no...they have childhood abilities that come as a result of innate physical youth.
Just as when you get older, you lose your eyesight, hearing, etc., in this campaign the children would lose their 'youthfulness' at age 15, when they become like the rest of the adult population. Call it the 'gift' of youth.
If they regularly work at improving these skills they don't lose as much of them, and as a result they start out with better than average skills when they reach adulthood. Using them semi-regularly (which is what I haven't decided on yet) may not cause them to lose them at all (perhaps if they improve 5% in a skill in a year, they may not lose anything, but instead gain 5%). But if they don't use them at all, or very little, they would lose 5% in a year.
They don't improve more slowly. They are given a gift (of youth) at the beginning of the game and it slowly drains away if not used. They never improve more slowly than they would normally improve in MRQ.
