I know there are rules in ultimate campaign, but what do you do about it yourself?
In general, I don't think that it can be handled for PC's. I have rather elaborate rules for infant, child, and youth NPCs and have since 1e, but I never apply them to PCs. They are more appropriate for keeping track of a PC's children or modeling important NPC youths or just having a generic stat block for children if you need one for some reason.
I find that there are generally only two ways PC's taking 'youth' mechanically works out. Either the player is punished for taking a young character concept because the young character is inherently inferior as a starting character - less mature, less knowledgeable, less skilled, etc. - and the player finds the rules, though 'realistic' unsatisfactory in some fashion.
Or else, taking a young character constitutes a loophole in chargen where the player starts out 'balanced' with characters of normal age but 'levels up' not only through the normal means by aging so that the resulting character is more powerful than they would have been had they started without the 'youth' tag.
Neither works. For that reason and others, immature PC's are thus banned as concepts in my game.
Basically, my method for handling NPC children treats their ability scores as being a percentage of their maturity, with Str, Dex, Int, and Wis starting at zero and going up linearly, and Con and Chr starting out at 1/2 the mature level and increasing proportionately with increase in age. Infants are two size classes smaller than adults, and children are one size class smaller than adults and have the Lithe trait (that lets them add their size class as a modifier to certain skills, making up for their otherwise low dexterity). Youths are the same size as their adult counterpart and lose the lithe trait. Characters are 0th level commoners until they qualify for a class, usually by obtaining at least a 9 in one ability score (my classes have minimum ability score requirements), and having appropriate background (schooling, apprenticeship, tutored, mentored, acolyte, etc.). At this point they become 0th level characters in the appropriate class. Zeroth level characters have -400 XP, and become first level at 0 XP, which usually requires a character spend about 1/4 of their youth in training (more or less depending on the aptitude of the student and the skill of the teacher). For humans, this means that they are generally 1st level at some point between age 10 (well before they've obtained their full ability scores) and age 20 (somewhat after obtaining their adult ability scores).
Considering that we've been playing for 4 years, and the game time has moved about 4 months, I can't imagine that playing a 10 year old under these conditions would be attractive. A typical stat array for a 10 year old male human PC under my chargen rules would be something like. Str 9, Dex 9, Con 11, Int 9, Wis 9, Chr 8. They'd be a lithe small sized character, and thus slightly more nimble than their low coordination and manual dexterity might suggest. That represents a human PC with a 14 Str, 14 Dex, 14 Con, 14 Int, 14 Wis, and 10 Chr at maturity. This otherwise prodigal child of prodigious ability for his age would probably be at most a 0th level apprentice. While they would be about the equal of most non-heroic adults, they'd be far inferior to mature PC heroes. I'd treat being say a 16 year old human as basically color, with no mechanical effect important enough to make note of for game purposes.
