no offense but that idea is not a good one. firstly, if you demanded to know someone's age for profile purposes of course they would lie and second, is it really so smart to put your age on the internet?
Not good. If they showed age. They would say, "LMAO, THAT KID IS SO YOUNG I COULD FLICK HIM OUT OF HOUSE" or "zOMG, YOUR SO YOUNG XD XD XD XD"
In past they used show age near log out and "Hi, Your username"
No. This would ruin many lives on ROBLOX. (Possibly)
For example, you told your new best friend you're 14. If this happens, many people will be dissed.