Here is a quick off-the-cuff houserule that I intend to use if the next generation of player characters are younger than 21 when they are introduced into play (due to the death of their fathers/older brothers).

Making a young character
(Basic skills: Sword, Lance, Battle, Horsemanship, First Aid. nc skill = non-combat skill)

Age Chargen
15: Basic Skills at 5. 1 nc skill 10
16: Basic Skills at 7. 2 nc skills 10
17: Basic Skills at 9. 2 nc skills 10, 5 skill points
18: Basic Skills at 10. 2 nc skills 10, 1 misc pick, 5 skill points
19: Basic Skills at 10. 3 nc skills 10, 2 misc pick, 5 skill points
20: Basic Skills at 10. 1 skill 15, 3 nc skills 10, 3 misc pick, 5 skill points
21: Basic Skills at 10. 1 skill 15, 3 nc skills 10, 4 misc pick, 10 skill points (i.e. a fully trained knight in basic rules)