Without giving out too much spoilers, there is a part towards the end of the Shadowbringer patches (either 5.4 or 5.5) where Urianger comments to another character that he was about the twins' age when he had his final growth spurt (apparently it is an Elezen thing that happens at about 19 - 20 years of age). He said he grew up from their height to his current height in a single summer.

I think we may just get a "timeskip" in a patch soon where the twins aren't seen for a while and come back at full height. I am betting they will no longer be as similar-looking either.