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 and training, 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".

Anyway, the point is that if Eren ate Reiner he wouldn't transform into an Armored Titan, and if he ate Bertolt he wouldn't become a Colossus Titan. Let's not forget that Eren already has the whole Reiss succession of Shifters inside him (First King included) plus Grisha, and yet he hasn't displayed any special traits or abilities besides the Coordinate. He wasn't even able to harden until he swallowed that "Armor-Braun" bottle, even though it's very unlikely that none of the Reiss Shifters possessed the ability (again, First King included).

And now that I mentioned the "Armor-Braun" bottle, that give us an idea of what Zeke was talking about.