I'm trying to design a character that is under minimum age (because I disagree with age limits to a race, as long as it's within the limits of ability to act). My character is a dryad that will carry a sprouting tree with her. She's very young and naive, but she's running, with her tree, for her life because her forest was destroyed (fits with the campaign). How would level adjustments work? how do you decide alterations for under age? (I assume less wisdom (because they have no experience) and str, higher cha, because children imply additional cha (ref not currently included, but I remember reading it. I'll do research and add a reference.)).
There's no official solution to your problem…
The DM's allowing two things the game forbids: a subadult dryad (Monster Manual 90-1). The Player's Handbook says that "[i]f you choose [your character's age], [the character's age] must be at least the minimum age for the character’s race and class" as per Table 6–4: Random Starting Ages (109-10). Likewise, a dryad has a Level Adjustment entry of —, so the game believes a dryad inappropriate as a PC.
This (ahem) leaves both questions for the DM to answer. However, the game can provide some assistance in adjudicating matters if you're willing to turn your head and squint.
…But Savage Species puts a typical dryad at maybe about LA +6…
Because there's no other official source explaining how to determine level adjustment, out of necessity we are forced to use the terrible rules in Savage Species for determining the dryad's level adjustment (11-24). According to that text, here's how a dryad's level adjustment works out:
- +1 LA for +3 natural armor. (11)
- +1 LA for unbalanced ability scores. (11)
- +1 LA for higher-level spell-like abilities. (13)
- +1 LA for spell-like abilities usable at will. (13)
- +2 LA for damage reduction that's not bypassed by a magical enhancement bonus. (17)
Thus, so far as I can tell, with the dryad's 4 racial Hit Dice and level adjustment of +6, the typical dryad is ECL 10. That's dumb, of course, but so is level adjustment in general, but you asked. (Savage Species doesn't provide a dryad's level adjustment because a dryad is just too difficult of a monster (22-3).)
In addition to other methods the DM may allow for reducing the creature's LA, ditching the dryad's spell-like abilities deep slumber, speak with plants, suggestion, and tree stride would reduce the above LA by 1; making entangle and tree shape not usable at will would reduce the LA by 1; and removing the DR 5/cold iron would reduce LA by 2.
(As an aside, in this DM's opinion, a dryad is in no way equivalent to any level 10 character, but while Savage Species overvalues many of the dryad's features, its ability scores and spell-like abilities are probably actually worth a level adjustment of +1 or an underpowered +2—at will speak with plants is a solid investigative tool and tree stride once per day is pretty much solo teleport several levels early, for example, even if neither will matter much past an average party level of 8.)
…And some guidelines for underage creatures are presented in the description of giants
The Monster Manual on Giant says
When a group of giants includes children, roll d% for each child to determine maturity: 01–25 infant (no combat ability); 26–50 juvenile (two sizes smaller than an adult…, −8 Strength, [and] −8 Constitution…); and 51–100 adolescent (one size smaller than an adult…, −4 Strength, [and] −4 Constitution…). …Except when otherwise noted, giant children are identical with adults of their variety. (120)
(This section is absent from the SRD entry on Giant.) Now, I know that giants aren't dryads (nor are giants humans or anything else but giants—thankyouverymuch), but, as the rules are otherwise silent on the statistics of children, these rules can provide at least some direction. With that in mind, given the lack of gradation within D&D 3.5 size categories and ability scores, ruling that an adolescent is one size category smaller than normal and possesses −4 Strength and −4 Constitution compared to an adult seems fairly reasonable. I mean, after all, the Player's Handbook considers ready for a life of adventuring a typical adult 16-year-old Medium human with full ability scores, so if the typical adolescent human is Small and possesses Str 6 and Con 6, that's a darn good reason for waiting a few years before turning that level of commoner into a level of warblade via a rebuilding quest (Player's Handbook II 197-8) and camping with strangers until he dies.
(I should make it clear that I agree with KRyan's answer that character age should be a player decision without mechanical ramifications, and I've had no issues with allowing subadult Small human PCs possessing adult ability scores into my campaigns. I will admit to some concern and discomfort as a DM in having horrible monsters attacking a child, but those feelings are mitigated somewhat after a child psion pops a few monsters like soap bubbles.)
Now, about lugging around that tree…
If the DM has already ruled that the subadult dryad can satisfy her supernatural ability tree dependent by carrying around her potted oak tree, that potted oak tree becomes her most prized possession… and that makes the dryad incredibly vulnerable. If separated from it by more than 300 yards, the dryad will "become ill and die within 4d6 hours" (MM 91), which likely includes if the tree's destroyed or if the dryad's enslaved or imprisoned. While the imagery of a warrior who fights while cradling something precious is evocative, toting around an obvious and highly flammable item which means the owner's death when it's destroyed is an extremely tough (ahem) row to hoe in D&D 3.5. Clear with the DM exactly how dangerous this is before sinking a lot of emotional real estate into such a character: be sure the DM views this as a serious attempt at playing a particular kind of character so that her end will be suitably tragic rather than offhandedly cruel.
Age penalties are bad for the game. You should just play your character at whatever age, assuming the group agrees it’s acceptable, and then just pick your abilities as you consider appropriate for the character. You’re playing a supernatural creature to begin with; it’s not like we can even guess much about what dryad maturation is like anyway.
And while you’re at it, the group might want to consider just ditching the aging rules altogether. They are awkward and arbitrary, and do not actually reflect aging very well at all. Is your character old and weak, but also learned and wise? OK, you can choose to make your Strength and Constitution low and your Intelligence and Wisdom high. You don’t need bonuses or penalties, you can just pick your scores appropriately. And someone else who is playing an old character that has kept in fine shape can choose to have high Strength and Constitution, and that character really doesn’t need penalties.
In the end, all aging penalties do is encourage players to keep their characters young, and arbitrarily punish those who don’t. Eliminating them may expand the variety of characters people are willing to play (and rewards those who would have done so anyway by eliminating unnecessary penalties), and it also gets rid of a few cases where aging can be exceedingly imbalanced (e.g. this kills venerable dragonwrought kobold shenanigans, and that’s a great thing).
Attempting to actually produce underage bonuses and penalties that are consistent with what happens with old age just produces nonsense or bad gameplay. Either you reverse the direction of aging penalties and bonuses (so a character who is not physically mature receives bonuses to physical scores—nonsense) or you apply penalties to both physical and mental stats (so a character is just strictly weaker than other characters for no good reason—bad for gameplay). Or you do something weird, where you try to apply tropes and stereotypes to the game rules (“kids are quick, right? so Dex bonus? and they learn fast, but they don’t know as much, so penalty to Int and bonus to Wis? children are natural rangers, what?”) and just produce a mess people will only argue about (bad for just about everything).
There were rules about 0-level characters in older versions of D&D, but they were all removed from 3.5, probably to streamline the system.
The manuals you could refer to are AD&D N4 Treasure Hunt, Greyhawk Adventures and D&D 3.0 DMG (pg 40: "apprentice-level characters").
You could try to port those rules to D&D 3.5, but it would be a heavily homebrewed system, and all the party should roll with it and start untill the DM decides you're all 1st level characters.
The important point is that you should not create any kind of serious penality for your character, as this will end penalizing the whole party, unless ALL the party agrees to take the same penality and all encounters and challenges get toned down to match.
Non-Official Possibilities Exist to Cover Your Question
If you accept non WotC/3rd Party content there are non-primary sources that cover this.
D&D Wiki has a "Homebrew" Child Template that does provide for stat modification. This site is not normally favored here and I personally do not like referencing their "homebrew" templates. Since homebrew templates are not official they are subject to DM approval which will vary across tables/DMs making them a less than ideal reference. However, there is no other 3.5 source that covers this so it's the best we have to offer in published text.
I also hate referencing Pathfinder in 3.5 posts due to the distinct differences between them but again the lack official 3.5 sources leaves us with little other option aside from "no you cant" (which I despise most of all). That said, there is a PF Young Template that also lists some stat changes but it is not nearly as thorough as the previous template.
If you were to use one of these I'd recommend the one from dandwiki but if nothing else it's a starting point for you and your DM to work from.