i just think it would be interesting!
Sure, but just because something is interesting, that doesn't mean you have a right to know it. I mean, it would be interesting to know all sorts of personal details about people, but for obvious reasons that doesn't mean they should have to share them with you.
If someone's age is relevant to their question - eg if they're asking 'I keep having night sweats, what could this be' or 'I've really gone off sex in the last couple of years' or 'I want to have a baby but my boyfriend says he isn't ready yet, who is BU here' - and they haven't given their age, then people can ask in the replies and the OP can decide whether to answer at their own discretion.
Age is just one of many, many things that might make a difference to the way people see things. Should everyone also add their ethnicity, nationality, religion, political affiliations, salary, location, social class and sexuality? Clearly not.