I always look at MM 1-5 through a kid's filter. It would have been fascinating to have played those games for the first time as an adult, with all that I know now.
(I guess MM 9 and 10 scratched that itch somewhat, although I don't consider them near the quality of MM2 or 3.)
I think I was a teenager when I really got into them, I think I was 16 or 17. Mega Man games are generally very good. Mega Man 2 actually ended up being my favorite of the bunch (despite no nostalgia). I think it's one of my most replayed games of all time!
Man I should start marathoning the games soon.