I am curious if most people know the exact ages of their friends? I realized that I think I do know most of them, of people I've known for a few years..there are some where I'm not exactly sure. I have definitely been asked how old I am--like many here, I look younger than I am and I turned 30 this year, many people are surprised and/or disbelieving..I had to take my ID out to prove it to a friend once! (I hate saying that as it sounds like a cliche humblebrag...)
I don't really mind saying my age, though it could be odd to be asked in some circumstances. With a closer friend, it wouldn't bother me at all, and I'm pretty sure that's a question that asked in my circle at about the same intimacy level as 'how did you meet your SO'. It doesn't usually come up cold, but more in talking about various subjects, though.
One thing I don't entirely agree with is the "it's rude because why do they need to know", well, we ask things all the time of people that we don't strictly 'need' to know..it's just conversation and information. That's not to say there aren't personal questions, and I can see how age would be one (though it's not for me.) I just don't really understand the "they don't need that information" reason for it being rude.