For the new 2011 version, twins Wilykit and Wilykat were the youngest members of the ThunderCats. Lion-O himself was a Thunderkitten in the first episode, but he is aged into an adult body during space travel.
Lion-O, voiced by Friedle, was at one time, the second crowned prince of Thundera but he was crowned the King of Thundera and Lord of the Thundercats. Unlike his original self, Lion-O isn't a twelve year old child stuck in the body of an adult.
ThunderCats originally ran from 1985 to 1989. The new series produced by Warner Bros. Animation began airing on Cartoon Network from July 2011.
For updates related to Thundercats animated series show and movie, go to http://thundercats.org/.
Of interest, check out 15 Amazing Facts About 'ThunderCats' from MentalFloss.com.