I didn't read it in wiki or anything, that's just how I always remembered it. Huh, go figure.
I wasn't suggesting you did, but when this convo first started I googled it and found other conversations where people said the same as you and even derided others for not knowing the capsule was damaged.
I think because there is a battle with the mutants in the beginning of the episode and the ship is badly damaged (that Panthro soon after patches up), and the fact that the ship crash lands on 3rd earth and is smashed to smithereens, you assume that things must have not been working right and is why Lion-O grew.
Also at the beginning Jaga says that the "Nobles" have assembled (ie: the other Thundercats), so I can only guess that they are already fully grown and is why they didn't age discernibly.