HAHAHA... This is hilarious. About 8 years ago, when I was in my mid-20s, single, etc, I hung out with Julie Bowen a few times (the mother on Modern Family). She was probably 30 or so at the time, and acted young. Now I'm married with kids and my wife and I watch the show. I was thinking the other day that it makes me feel old that Julie can now play off being a mother of teenagers. I remember her as someone who was very far from that and that made me feel quite old.
From a physical perspective, I stopped feeling young when every day became painful to run because of stupid niggles, and I couldn't just head out the door and hammer. Guess that was around 30.
Having said all of that, being one of the younger dads in my town, and hanging around with a 2 and 3 year old quite a bit, I do still feel pretty young most of the time.