as an actual young med student (19) who goes to school with a few other med students who are quite young (17-21), i can say that there is no place that i'd rather be than here. we are all very serious about our education, but it's not like once you enter med school, life ends. in fact, i can say for sure that i have had as many memorable experinces in the last 2 months as i did in all of undergrad. it's all what you make of it.
SDN is very good about maintaining a group mentality, which i had discovered just last year. there is a lot of negativity and criticism towards people who don't venture to other countries or lifestyles because somehow, you can't be a good doctor or even a med student unless you're thoroughly satisfied that you don't want to do anything else and be anywhere else. and that in itself is pretty narrow-minded.
in terms of maturity or social awkwardness, i assure you, my classmates constantly (almost irritatingly) forget that i am not yet 21 because i talk and act exactly like they do.
my advice to young applicants though, if you are confident of your reasons to pursue medicine as your career, and have no doubt of where you want to be 10-20 years from now, do it. don't let people on the internet who don't even know you detract you from doing what is best for you. but approach this process and the rest of your lives with humility.
good luck you guys.
