This thread is mostly young adults feeling miffed about not being treated as adults on the basis of an easy and entirely understandable mistake of being mistaken for someone less than 5 years younger than them. People want to be treated respectfully, and see being called as a "kid" as disrespectful, but firstly, even if you tell them your real age, they will still think you''re a kid, and secondly, people aren't necessarily being disrespectful for calling you (or me) a kid. Its just their perception due to their age, as we might call them old.
In terms of not being taken seriously for young looks, you will be taken seriously by employers because they don't care how young you look, they just care about what you can do for them. If I go out with other university students during the weekday people might ask if we got let out of school, but that's because most older people don't bother to notice if someone is 16 or 21, there's only 5 years difference, how can they tell? People our age seem very focused on the idea of who is older and younger within that narrow 15-22 window, but its just a primal desire for immediate respect. Immediate respect is not necessary, if you don't like someone, you can talk to someone else.