I've got the same problem, caused by a delayed puberty, but started when I was a kid. The thinnest and small one at the primary, secondary etc etc school.
Sorry I will explain my position cause I was thinking about that today, and I was thinking about opening a topic about it...
I never cared so much about the 'have you the legal age to buy alchool?',(or everything else) sometimes it happens even today 'yes I've got it since 10 years, maybe I'm older than you, bartender, you that you're asking me this thing...' etc.
With the strangers and acquaintances is boring, but in some way I've learn to use it, like 'Ok you're treating me like a kid, so if I'm not interested in talking with you I use this as an excuse to avoid you.' It's not something complicated, maybe was an inconscious psychological defense, to me. I simply act like 'I don't care'. Or I joke with these people, if I'm in a good mood.
Is not only when they're talking to you, sometimes is like they're
watching you, like underestimating you. Even some friends, who are immature and you're like 'Excuse me?' Honestly I use 'the method' also with some friends...
My problem, was on personal/private life. Luckily I had a strong personality in the past, but for example, during the high school was not easy being 'a kid' in a class of young men. They never bullied me because of my personality (was not so easy to tease me... And don't forget the revenge... XD) ...But I was aware they would humiliated me if I was weaker. But i was kinda strong in my way and they were my friends...or civil.
Today I'm in your position. I don't want a younger partner only because a question of feeling. I know there are mature guys around 2oyr and immature 30,40 etc, but... I'm a little afraid. And the main problem to me is that I feel weak from a sexual point of view, even if I'm not virgin (but I still don't have a great experience heheh).
The girls, gosh, they want older guys and I'm always like the young brother... The guys... It's easier with the guys, cause they care less, but I feel weak, like being 'the boy'...