Board Thread:Questions and Answers/@comment-25139993-20180830163953/@comment-995426-20180906042342

Since the 2003 series followed the Mirage formula (relatively) more closely, I think the turtles were indeed teenagers, but were indeed strangely buff.

The unofficial webcomic  (an alternate universe from Mirage but whose art style is inspired by the 2003 series) supports the theory that the turtles hadn't finished growing as they aged.

Mirage itself ended up not going this direction, instead making the turtles in their 30s look the same they did as teenagers.



But they still theoretically had a long lifespan. Below, from left to right: 8-year-old Leo, 100-year-old Leo, 45-year-old Leo, 15-year-old Leo.