ThunderCats S02E12 - Time Switch

While burying one of the time capsules that transported the Thundercats to Third Earth, it smashes apart and Lion-O gets caught in its gases, making him younger and younger by the minute. The other Thundercats must find a way to reverse the process and realize the Cave of Time is the only way, but they must get Lion-O there before he "vanishes" from existence. Things get worse when the young Lion-O and Snarf are captured by the Lunataks.