You can look at this from different aspects.
Is it appropriate for the interviewer to ask for your age during interview? No it is not appropriate.
Should I reveal my age voluntarily ? Up to you, there is nothing wrong in it.
I personally do reveal it whenever the interviewer ask me to tell him more about myself, I give him a quick person background and my age is a part of it.
Why do I do that? I do it because, it gives the interviewer a level of confidence about me. When the interviewer sees my resume and background he always assumes I am 8 to 10 years older than my actual age. By revealing my real age I am telling him that I am someone who has held position beyond his birth certificate age.
To summarize, If you think your profile is beyond your age, go ahead and tell them your age as it gives them a level of confidence about your character.