There are three factors to a person's age:
1) His age based on time, this is from the day he is born.
2) His intellectual age: This is based on the faculty of understanding divinely gifted to him by Allah. Sometimes his intellect could be more advanced than his actual age, sometimes it could be appropriate for his age and at times his intellect is underdeveloped and not on par with his age.
3) A person's academic age: This begins when he starts acquaring knowledge.