If something lasts forever or even if it just feels like it’s going to last forever, you could call it eternal, which means that it goes on and on and will never change or end.
The Fountain of Youth is a mythical spring of water that rewards its bathers with eternal life, free from sickness and death forever. The Latin root of eternal is aeternalis, translating roughly as “of great age,” and eternal things are really, really old. Eternal can describe an abstract idea that never changes, like the eternal truths of war, or it can describe an annoyance that you wish would end, like the eternal boredom you feel watching a really dull movie.