It’s not necessarily what you rap about, but how you rap about it. There’s many long time vets that rap about what they rap about and still sounds nice, from recent vets like Slim Thug and Paul Wall to the further back folks like Method Man, Onyx, and MOP. The problem is when you sounds like a relic. KRS-One to me has sounded like a relic since after his 2007 album. Same thing with a lot of 80s rappers in general. Melle Mel sounded terrible on his Eminem diss. MC Hammer was brutal trying his comeback in 2001. As long as your flow sounds as close to your peak as possible, age shouldn’t matter.