I was hoping maybe I could find a manga where the main character is super smart or needs a job and hides their age so they can do something? Like how in Black Haze the Black Magician hides that he's only a kid and how in Hayate no Gotoku Hayate hides that he's not 18 yet to get that bicycling job at the beginning, but you know, more focus on that specific point?
(What I really want is someone around high school age or younger that super smart and becomes a teacher -like Negima- but hides their age... that's kinda specific though soo)

OMG I LOOVE BLACK HAZE
I also love secret identity manga, but I've only got two suggestions for secret identity concerning with age>>
Penguin Revolution
Crash!
Both about underaged girl working as a manager for an idol/ idol group.
Well~ if you want more manga with the protagonist having a secret identity, i could suggest some more. Although, it won't be related with age.