How old do you have to be to be a knight?

If your actual question is ,"How old do you have to be in order to be a knight?", then I have an answer for you. I guess if you are imaginative, you could be a knight fighting an evil dragon in your mind, and you could do that at any age! But if you want the logical answer, you had to be a strong, tall man, which probably means he has to be young as well. So they would have to be at least in there 20's.