At my age (45), it's when I get carded for 21, or even 18. That used to happen a lot more than it does now, but it happened as recently as 2 weeks ago ...
Since I'm into cutting and definition more than size, I get away with looking younger by having a flat sixpack and wearing T-shirts & "young" clothes. ('til anyone notices my white stubble).
Maybe the BEST compliment I've ever gotten, was last summer when I walked into a small town cafe in Parkfield, CA, and the entire cafe got up and screamed and whistled at me. My wife had passed around a shirtless picture of me, evidently.