Observation: If I look at certain things I did when I was 15, I'm often extremely embarrassed by them, and have been since I was about 20. But I can't think of anything I've done since I was 20 that I feel similarly embarrassed about.

Possible explanations:

1. Personal growth/change rate stalled around 20; I stopped "advancing" in ways that would lead me to know enough to be embarrassed about more recent things. Or at least, I'm just less different from me-at-20 than me-at-20 was from me-at-15, and so more easily feel justified in post-20 actions (e.g. due to fundamental attribution error applying more cleanly)
2. Propensity to store embarrassing memories decreased around 20; I still do just as much fundamentally-embarrassing stuff but I'm less good at remembering it
3. I've hit some kind of "embarrassment critical threshold": a point in personal development where I outgrew or suppressed most impulses that would be embarrassing later. Also counts if I became better at predicting what would later be embarrassing.
4. Generically my life has had less variance per day after 20, or the moments in it have been differently-selected, resulting in fewer opportunities to do embarrassing things