Aging in comic books is a rather arbitrary affair, more closely connected with plot than with the passage of time. Most comic book characters don't age at all, but when they do, it tends to either because of a time jump in their title, because of time travel shenanigans, or in a few rare cases, a new creative team forgets how old a character was to begin with. Tim Drake, the third Robin, has been in his mid-teens since his first appearance, despite the aging of the first two Robins, and there's a reason for that.
For the most part, comics companies use the sliding timescale to keep their characters young so that the characters can continue to appeal to a younger audience. While people of all ages do read and enjoy comic books, the target demographic tends to be a lot younger. He may have finally gotten a new identity (and a terrible costume to go with it), but as one of the founders of Young Justice, and the current longstanding member, the message is clear, Timothy Drake is for teenagers, and that's not going to change anytime soon.
However, it's not just the need to keep Timothy Drake's appeal to younger readers intact that's responsible for his lack of aging. Timothy Drake's future has been teased, multiple times, and he has the potential to be one of the DC Universe's greatest heroes, or one of its greatest villains. Timothy Drake is Schrodinger's Robin, locked in a box of eternal youthfulness because neither heroism nor villainy is as compelling as a teen hero with the potential for both.
Timothy Drake in his earliest appearance used his own detective skills to deduce the one thing that most of the DC Universe has been unable to do, Batman's secret identity. Tim Drake needed no traumatic origin or a sudden infusion of powers to bring into the superhero life, only his prodigious intellect, detective skills and a determination to make the world a better place. When he was first Robin, Batman himself repeatedly acknowledged that Timothy could one day surpass him as a detective. Given Batman's significance to the DC Universe, that's high praise.
There's a darker side to Tim's future as well, one we've seen quite literally in Geoff Johns' run on 'Teen Titans.' In the 'Titans Tomorrow' story arc, we see that in the future, Timothy Drake has taken up the mantle of Batman. He's a much darker Batman, ruling Gotham with an iron fist, and using Joe Chill's gun to kill Batman's biggest villains. The initial run of DC's Detective Comics following the 'Rebirth' relaunch, written by James Tynion IV, explored this idea further.
The young Timothy Drake of the present is making plans for a safer Gotham, using extreme measures of surveillance. A look at the future shows where that leads, Gotham is in a surveillance state, ruled by a tyrant Tim Drake wearing Batman's costume and willing to keep the peace by any means necessary.
If Batman is the peak of humanity's capabilities, then Tim Drake is the story of humanity's potential. What makes his story so compelling is how far Timothy could go, given enough chance, in either direction. The fact that he has the potential to be the DC Universe's most dangerous mind, save for his friendships and his family, is a question that might just work better if left unanswered. Timothy Drake should remain Schrodinger's Robin for as long as possible because his story is about the future he can attain, once he actually attains it, then his story is over.
Timothy Drake can be found in the pages of 'Young Justice,' which releases on April 8, and 'Robin 80th Anniversary 100 Page Super Spectacular,' releasing on March 18 on comic bookstores.