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 'If Olou and Petra had lived, they may or may not have gotten married'

 'Levi '''is in his early 30's. Hanji is 2 - 3 years younger than him. Erwin is in his late 30's. Olou was 19'.'''This means Petra, Gunter, and Eld were probably 19 too, since the manga implied that they graduated together. They were so young.

Oh and also,

 'Levi is already in his 30's, so he's stopped growing'

 This is based from Isayama's Autograph Session