also. The mutant aspect makes me want to wonder exactly what “teenage” means for them

If their aging is fully humanoid, and they aged for a number of years in the “teens” after being mutated… does that replace, or combine with, their prior aging in turtle form, and how much aging was that.

(probably not much, since they were described (at least in the version I watched) as four baby turtles)

However if their aging was inherited from the turtle side of them instead of the human side, the species of turtle is very relevant. The most common pet turtle species is the red-eared slider which lives only to 30 or 40 years at the very most, so teenagers of that species would likely be past their midlife crisis. A species such as the Greek tortoise, on the other hand, would likely be reproductively mature by that time but still extremely young by species standards.

(As the original “four baby turtles” were in a bowl of water when they fell into the sewer, they were probably aquatic turtles and not desert tortoises– unless they were being very badly kept.)