From experience living with my cousin who got a puppy when he was 19.
- take that dog out, and bitches will come flying out of nowhere
- good companion, that applys no matter how old you are
- good reason to get outside regularly and find cool new places to walk
- the novelty wears off real fast
- it WILL piss you off, puppies usually spend a good deal of the day acting like 5 year old pagant queens.
- it WILL piss off your roommates. Eating food it finds, chewing shoes, shitting everywhere, theyll be more pissed than you are since its your responsibility
- harder and costs more to rent. Mainly for above reasons
- Most owners get too busy and stop walking, don't maintain a consistent feeding schedule, and generally at 20 you aren't responsible enough to own an animal that requires that much care.
If you are set on a pet, maybe try to find one thats easier to manage. I have a snake that takes feeding and cleaning its shit. I have a friend that has a chameleon and it doesn't take much either. Find a pet that doesn't require tons of time out of its pen/tank/cage and close to constant supervision.