Even if naruto gets old, he will never be lacking, he has regenerative powers/the bijus so at the age of 90 he could sitll have the energy of a 12 year old. The only thing that might be able to kill him is probably a clan wide disease or something. Like how hagoromo passed on his death bed.
Point is, once naruto get his other half of kurama, he'l be peaked at powerups. Unless there another sage mode for him. As for sasuke, as long as he's with orochimaru, there will always be power ups, im not fanboying but its how its going to be. Orochimaru will hon sasuke's powers to perfection for him, and maybe eventually take over him. WHich is highly unlikely but its a possibility.
Sasuke and naruto having to have to fight might end up being on these terms. Where naruto might have to save him fro mthe clutches of orochimaru. (not as detailed as i wanted it to be but i feel like eveyrone's smart enough to know what im talking about.)