system
A chat on subreddit /r/NoStupidQuestions
human
I have a youngish-looking sister.  For the first part of her career, she looked like a kid visiting a parent at work.  Then she got a new car, and on that day when people asked "How are you?" and she said "Just got a new car!  I loved the old one, I guess we all love our first cars, but I'd been driving it ten years and it was time."  She could see people working out that if she'd been driving for 10 years then she must not be a teenager.

After that, she'd say cheerful stuff that gave away her age whenever she did the "How are you?" thing for the first time with somebody.  "Birthday coming up; it's the big 3-0."  "My husband took me out for the most romantic dinner; it was our 5th anniversary.  He's such a sweetheart."

And it's still a thing!  She's a grandmother about to turn 60, and people talk to her like she's in her 40s.  A while ago during a discussion of 9/11 someone said "For my generation, the 'where were you' thing was the space shuttle blowing up."  She was in grad school when the space shuttle blew up.
gpt
It will always be a thing! But at least after a certain point people believe you are an adult.