Board Thread:Manga/@comment-29012123-20170511142229/@comment-28958062-20170516222230

Age could be a factor, both Grisha and Zeke got the serum later in their lives and both titans have abnormal large torsos.

I suspect the older the candidate the more abnomalies arise when they inherit the shifter power, that's another reason on why they use younger people as oppossed to older ones, too many anomalies would result in a poor use of the shifter power.