I like upperclassmen boys.

But I get the feeling when I talk to older people that they don't think that I'm cool. I don't get a vibe that they don't like me but a vibe that they don't think I'm on their level of maturity or knowledge.

So... How do I get them to think I'm cool and not just trying to get invited to their parties? Because there's not that much difference between 18 and 21 really.

disgusting.

__________________
i also like hobos, riding dinosaurs with my best friend jesus and eating glitter.

actually theres a HUGE difference between 18 and 21. you probably wont be able to understand until youre my age tho.
im a completely different person than i was when i was 18. i look back on things i did back then that seemed cool at the time and roll my eyes now.
your mindset changes a great deal, as do your interests in those 3 years. it might seem like there isnt a gap there but there really is.
but, my best advice is dont try to impress these people. when i see little 18 year olds doing shit like that its just stupid. ive been there and done that so i see right through it. just be yourself.
most of the people i hang out with are 5-8 years my senior. they like me because i never tried to impress them, i dont talk about things i dont know about to impress them, i dont start drama, and i dont talk shit. if you do any of these things chances are someone of maturity is probably just going to think youre a tryhard and laugh at you.
for the record tho, most 21 years olds arent very mature either.

Why would you want to hang out with people who wouldn't accept you unless you went out of your way to impress them?

Maybe you aren't on their maturity level, did you think of that? If they don't think you're cool, there's nothing you can do to "make" them think you are. You shouldn't try and "make" them think you are. That's like being fake just to try and impress them. Being fake and trying too hard to impress people is automatically not cool. Just be yourself.