go2bizSkool
at my former place of employment, there were several layers of job titles before the first real manager position (where you would conduct performance reviews, give people permission to take days off, etc).
the people in those layers often led projects or initiatives but did not have the authority to give a raise for example. I guess that is what i was defining as a "real" manager, and I can't see someone in their mid 20's given that authority, but that could have been only at my workplace
i'm 25 now. been a finance manager since i was 24...i'm a bit of an odd case though since i started undergrad at 15 and graduated by 19...so it took me 5 yrs of work experience to get here.
i NEVER tell ppl at work how old i am and if they ask i'll just say late 20's and never get into detail (i look older anyways, and i'm asian so ppl can't really tell). the ppl i manage range from early 20's fresh out of school to mid/late 30's...so it's not a terribly old group, which makes my age less of an issue and my job slightly easier.
i truly do think management experience is great to have for business school though. i've learned A LOT from managing ppl, so now i think i have a lot more to contribute to an mba class and i can also learn more too. but of course, it's tough for most ppl to get it before going into an mba due to the timing.