In the Teenage Mutant Ninja Turtles 2012 cartoon, Season 3 ends with the Earth destroyed and Season 4 sees the Turtles and friends go 6 months back in time, adventure in space then return to Earth and avert its destruction. Their past selves then leave the Earth with the Fugitoid and the Future Turtles & co. (whom we follow) then stay on Earth, taking their place. What happened to those sets of teens who went to space?
This was never explained in the series. One can only guess they had a space adventure and possibly settled down with the neutrinos. It's likely that writers didn't have a good explanation to close the loop.