How old would a teenage turtle really be?

[neatoman]

Human beings typically reach sexual maturity in their teens. Red eared sliders, cited in Mirage as the species the TMNT started out as reach sexual maturity around age five, which is presumably their equivalence of teenage years.

https://en.wikipedia.org/wiki/Red-eared_slider

Young turtles may carry out the courtship dance before they reach sexual maturity at five years of age, but they are unable to mate.

So assuming that a 15 year old turtle has been sexually mature for ten, they're like what? 36 by human standards?