Age and Generation are assumed to be linked for character creation…
The fifth edition makes an explicit assumption that the members of a coterie will be of the same (or at least very close) generation; see under “Age and Generation” in the character creation section, on page 139:
For game-balance reasons, all the characters should be of the same generation.
For this reason the book doesn’t explicitly support what you’re asking: there’s no Merit or other equivalent to the Generation background of previous editions, or the ability to increase Blood Potency with bonus points as there is in Vampire: The Requiem.
(You can increase Blood Potency with experience points, but you only get XP at character creation if creating an older vampire who likely already has a higher Potency, and the three tiers presented - Childer, Neonate and Ancillae - either get too few XP to raise it, or only enough that they’d spend nearly all of it on one extra point. This wouldn’t change your generation in any case.)
Instead, the default idea is that you choose the age and generation (together) for the coterie as a whole, as per the character creation summary on page 137 (under “The Sea of Time”).
…but there’s no reason you can’t do this anyway.
The easiest answer - with Storyteller approval, of course - is to just give the character a lower generation (along with the corresponding minimum Blood Potency, and all the advantages and disadvantages that carries with it). The text on page 139 goes on to say:
If equal power among player characters doesn’t concern you, feel free to play a sire and their childe, or any other combination of Kindred that intrigues you.
And, as noted in the chapter on “The Blood”, on page 214 (at the end of the section on “Generation”):
A vampire’s generation does not necessarily indicate their age. An elder might have sired a vampire of the 10th Generation when Columbus sailed; a methuselah could have begotten a vampire of the Sixth Generation last year.
So lore-wise you can definitely still have a lower generation freshly-Embraced Kindred. Just assign them the lower generation, without the additional experience points - those represent age and experience, not anything supernatural (as noted on page 151). With lower-generation vampires even rarer in V5 than previous editions, though, you’ll want to put some thought into why such a fledgling would be hanging out with other more common childer, and not being watched over by their sire or other more powerful vampires.
The main consideration from a balance perspective is long term. Blood Potency 2 is good, and will make a character more powerful, but that’s basically just 20 free XP. The much bigger advantage is that the character will be able to increase their Blood Potency beyond 3, assuming they live long enough and don’t mind spending a lot of their XP on the trait. But that brings its own problems - primarily, the character will have much more trouble slaking their hunger, and the severity of their Bane. (Note that errata released in the free V5 companion increases both the dice granted for a Blood Surge and the severity of Banes by one for all levels of Blood Potency, making high BP come with bigger drawbacks and rewards.)
If you want to balance this rules-wise somehow, I would first suggest limiting generation to no lower than 8, as in previous editions. That’ll minimise the impact of the above, and fits nicely in line with the limits of the old Background.
You could also use one or more of these suggestions, which I’ve based on systems I’ve used in other games (I’ve not tried this in V5):
- Restrict the low-generation vampire’s choice of starting Merits to ones which represent their situation; or you could create your own Advantage as part of a chronicle-specific loresheet.
- Give the character an “XP debt” equal to the cost of raising their Blood Potency to 2 (20 XP); they have to pay that off before they can raise it any higher. Story-wise this might represent the greater difficulty of mastering more potent blood, or overcoming a probably much older sire’s desire for long tutelage.
- Give the other players something for free to offset this advantage. The easiest example would be 20 XP, representing the fact that they’re old enough or otherwise talented enough to be entrusted to look after a fledgling of young generation.