Back in the anime continuity 765PRO, Ami and Mami did their middle school homework in the office, while Azusa have already graduated from college. That seemed a little at odds with the narrative of the pipeline. So, AKB0048 comes about, and, according to Evetaku, Makoto is the oldest in the 76th generation at 16, while Nagisa & Chieri are 13. Such spread corresponds to what we generally see in AKB48. For example in Karen Iwata's 12th Gen, almost everyone was born in 1997, except Tomu Muto, born in 1994. Okay, great realism, I suppose (with the exception of Konata, obv.). Still, I think it would be kind of nice if they had a late bloomer, you know? Perhaps they even do, I'm just not enough into it to read a gazillion of Wiki pages.

UPDATE: Why scan Wikis if Bing or Google can do it? Apparently the oldest member of AKB48 is... guess who... Mariko-sama (currently 24 years old IRL and still on stage with Team A). I should've known. Major spoiler: Tsubasa was a member under her name previously, and she's not a spring chicken. Wotas adore her both in the anime and IRL.