I don’t understand why people are so impolite as to ask you your age. I suppose they do that because your appearance resembles that of a child, and that gives them permission, or so they think, to ask a question they wouldn’t ask of anyone else.
Normally people don’t ask adults their age. I should correct that. Normally polite people don’t ask adults their age.
By assuming you are likely a child they give themselves permission to ask that question.
I don’t want to reduce this issue to merely one of good manners, but there are so many instances where people could avoid insults by simply keeping their mouths shut.
The proper thing to do if one is interested knowing more about you without asking your age is to ask questions they would ask of other people. For instance, do you live locally? How long have you lived there? Did you go to school around here? In the process eventually they can get an idea of your age. But only after they have gotten to know you a bit would it be appropriate to query about your unusual appearance for your age.
I am the sort of person who in general will not ask personal questions. Certainly not of people I have just met. Why people believe that it is any of their business how old you are is beyond me. But it is particularly poor manners that that would be among the first questions they ask.
I wish people would spend more time putting themselves in other people’s positions.
Bottom line, that is what good manners are about.