What is a good age for someone to start shaving their head?I don't know what is meant by "a good age". There isn't any specific age that's better or worse for shaving one's head than any other, as far as I know. Some cultures and religions have rituals that involve shaving the head of a child as young as 1 or 2 years old.
Shaving doesn't harm the hair any more than cutting it does, so shaving one's head won't affect later hair growth. As long as shaving is done carefully, it won't do any damage to the skin either. As far as I've been able to determine, there are no permanent physical affects that result from shaving, so there's no physical reason to limit headshaving to a particular age.
That said, there may be social and cultural reasons not to shave one's head at a given age. Particularly, children under 18 who are living with their parents may live in situations where their parents do not wish them to shave their heads. In situations like that, I generally advise children to abide by their parents' wishes; I don't think a hairstyle is worth the family discord that might result.
Once a person is an adult, I think they should be free to make any and all decisions about their personal appearance. Also, children who's parents do not object to headshaving shouldn't have a problem.
I've had emails from kids as young as 12 that shave their heads, and adults in their seventies. Nearly all report positive responses from family and friends. There is always the possibility that one's peers might not like the look, and even mock it. For younger children, the potential for social stigma might be a risk they don't want to take, and it is possible that teasing might result from shaving one's head. Generally speaking, though, it's not normally a problem.