peewee, I think Iker Casillias, when he made his breakthrough at Real was only 18 or 19 but he kept Bodo Illgner, a world cup winner with Germany, out and made some very outstanding saves for Real in the CL (year 2000 if I am not mistaken). He has been outstanding for them in the La Liga earlier on when he helped them scrape 1-0 wins.
You are mostly right in saying that 'keepers get more stable and mature as they get more experience and grow older but there are some exceptions. Petr Cech is only 22 but he has been really outstanding for Chelski this season and pulled off some saves which earned points for his team.