Board Thread:Manga/@comment-31940900-20160618204115/@comment-5774380-20160628124634

I noticed that Reiner and Bertolt's Titans didn't age, but I attributed it to Isayama's evolution. Both of them appeared at the start of the manga when many things hadn't been thought through, and Grisha's, Frieda's and Eren's forms (the original in Eren's case) appeared recently, when the manga was already pretty advanced. In all three cases, especially Eren's, their changes can be associated to the effects of aging, which Isayama maybe hadn't taken into account at the beginning.

But it could also be because mindless Titans have emaciated bodies and Shifters are well-toned as you said; although it seems to be a general effect and not exactly "gaining the traits of the Shifter they eat".