Now, I don't really think this is true. I don't even think I seemed like I was in my 20s through most of my 20s, much less now that I'm approaching the back half of my 30s. And I know he was trying to say something complimentary ... so I laughed it off and continued the conversation.
But, my actual reaction was more like, "WTF, dude, I've LEARNED these years. Don't take them away!" I guess probably there will be times in my life when I take it as a compliment to be taken as younger than I am (though mostly, I find the age thing pretty neutral), but it was interesting to me to feel so shortchanged by it in this case.