I have a house rule on this (only used for NPC characters since I have given them extra disadvantages, same as a normal human of 12 would have in comparison to an adult).
I calculate physical age at around 1.5 times that of humans, so a 12 year old looking elf is actually 18 in my games. I reduce their stats in accordance, for example -10% to all stats, an extra -5% to strength and toughness and a +5% or -5% to fellowship because they are children (depended on the situation, asking for help +5%, commanding an adult -5%). If they enter puberty (so above 12) the extra -5% to strength and toughness is sometimes left out. Wounds I also do times 0.5 (child) or 0.75 (teenager), rounded up.
You also start out with a number of skills and talents that you will have learned in your childhood, plus the skills and talents of your begin career. Some are physical but most are learned, for example I always let them have excellent vision and night vision, but coolheaded/savvy and aetheric attunement/SPW (longbow) is debatable. Normally I give them coolheaded/savvy but not aetheric attunement/SPW (longbow). Somethimes for wood elves I give them SPW (longbow) if they are known to have a talent in this.
Depending on their age I give them some of their starting skills and talents, for 12 I would think between 2 and 4 skills and talents. I have no fixed rules for this, I just look at the style of the game, their careers and the party role they will have.
For example I had an elf in a game that had a young sister (around 12 thus 18 elf years, kithband career) that was kidnapped and it was a long road to save her and get her back home. I gave her savvy from her race and outdoor survival, perception and scale sheer surfaces from her career. During the campaign the PC's gained a lot of xp and if I advanced her accordingly she would have grown up too fast. I gave her around 100 xp for every 300 of the party. I started with concealment and silent move (when combat started she would scream and disengage, after this she would hide). I gave her +5% to intelligence and agility since she used those most often (so she only had -5% penalty on those, instead of -10%). Then dodge blow, so she could also dodge before hiding. At the end i gave her heal so she could help the party after a battle. I made sure there was no way they could keep her on adventures (because of elven society and her overall weakness in comparison to the party). I felt she would advance too quickly for her years if I kept her with the party (she already had 7 out of 10 starting skills and talents from her career, excluding the one free advance, 4 advances learned in 1 year out of 11 learned in 16*1.5=24 years). Even if you say that you learn a career from 10 to 16, it is still 6 human years (6*1.5=9 elven years).
Physically I count them as adults (and give them stats accordingly) if they are 24 (16*1.5=24), but play them as more impulsive, less composed and overall more a mix between humans and elves. As part of elven emotional maturity is done the last 6 years (starting age human is 16, elf is 30), in my games at least.
As for emotional maturity during childhood, I played her as a very wise 12 year old human child. She had the same (sometimes irrational) fears and playfulness, but looked more long-term. For example, she once screamed in her bed in an Inn, the party rushed in ready for combat and she was afraid of monsters under the bed, it turned out to be nothing. Yet she also understood, better than most children, why some decisions were made regarding her safety, this did not prevent her from sometimes sneaking off to play with some human children etc. On the other hand, one time the party was in trouble and when a PC and her found help (she was slightly wounded and in tears) I gave the PC a +20% bonus on this check. So she was not always a burden and provided great role-play opportunity.
I do not recommend having a PC with these stats, but I really liked having an NPC attached to a PC like this. I did feel that If I did not use time skips it would be even more strange how quick she progressed for her years, while it sometimes looked strange that she progressed very slowly in comparison to the PC's. With elves this is even more pronounced since they are the slowest aging race from core rulebook (starting age 30, dwarf and halfling 20, human 16). I noticed I could not do too much time skips because the humans in the party would have aged too much. With human children this is much easier.