I'm 20 and it's whatever. I barely even think about my age. Physically, I don't think I look as much like a 16 year old anymore, but that's a tough call. My genetics struck a fountain of youth there or something. Mentally, I'm a cynical son of a bitch that has hated everything since I was a teen, so nothing has really changed there to make me think I feel older or younger. Financially, I have taken on more responsibility, paying my share of rent and other bills. I also have to worry about paying my college tuition and organizing around my parent's inability to offer any aid whatsoever, which has made me feel a smidgen older, I guess. With that in mind, I also haven't lived with my parents for several years, which has helped alleviate any potential early onset arrested development since I've known my only chance of finally being in a situation I want to be in is to get there on my own. I also have to deal with the state to handle my own insurance and other needs, which is a huge pain in the ass and something I don't wish on anybody my age.
I would guess feeling a certain age is mostly a matter of self sufficiency. Having a job, going to college primarily on my own support, and otherwise primarily supporting myself, I'd say I'm at around where I should be at 20. Those of you who feel younger than you are I'm guessing still live at home and/or are otherwise not primarily responsible for supporting yourselves. If you do bear these burdens and still somehow don't feel that you're around the age that would be making these decisions, I would guess feeling younger is more a desire than an actual feeling, wanting to cling to youth over facing the realization that you've entered adulthood and it damn well isn't going anywhere.
Trust me, I took one semester of psychology in college to get my social science credits; I know what I'm talking about.