For some people that started in high school
Yeah, in that case, I've been flabby since I was 17.
But on a less serious note:
Popping open the whatever music streaming service you subscribe to on Friday, looking at the new releases, and saying "I'm not tryna listen to any of this shyt."
The first time you get invited out, and start thinking of polite ways to talk your way out of going, because you'd rather kick it at home.
The first time you realize that you've gotta schedule shyt weeks in advance, damn near with email invites to get as many of your friends together for an outing.
Realizing the above is probably only happening a few times a year, even if y'all are all in the same city.
Eating something and thinking to yourself "I'm gonna regret this tomorrow."
Realizing you not only have a routine to avoid hangovers, but can tell in the middle of drinking when you need to initiate it or not.