Girls love to say this shyt. They're either complaining about how old they're getting or telling you that you're old. I just let girls know that I'm in my prime and don't have an expiration date. If they tell me they're old, I'll tell them, "you're only as old as you feel". If it persists, I'll tell them that they're hearing their biological clock ticking or to stop complaining and stop trying to bring me down.
I'm convinced that girls do this to project their feelings onto men to make the men either commit to them, make the man feel like he's old too, or bring him down so he doesn't start to stray. My buddy's gf is just like this. He told me about her and when I finally met her, the central theme of all conversations revolved around age and getting older -- it was really fuct up.