we're only really now getting rappers making music into their 40's. over the next 10-20 years i think we will see more cats like Nas making great music well into their 30's & 40's & more. the biggest thing will be getting over the need to "stay relevant" and cater to the youngest audience. there's cats in their 30's now acting like they're still 23 or younger. somehow i don't think Cheif Keef is going to transform into Jay Electronica when he reaches his late 30's if he's not in jail.
there are a few MC's i would say are still making great music in their late 30's & into their 40's some are even making some of their "best" music, Tech N9ne, Nas, KRS continues to drop great music, some of the Wu, Redman is making great music, Big Boi, Pharoahe Monch, not all of them have to make "grown man" shyt but if they know their audience and what they want then they can make dope music.