Over 60 is fair game to let the gray in.
Before 60, it really depends what your natural color is and how much gray you have.
I’m 38, medium brown with about 5% gray. I dye every 6 weeks with a demi color gloss. It doesn’t give full coverage but blends the grays nicely and fades them into highlights. There is no harsh line either.
As I get more and more gray though, I will probably have to come up with a different plan. Dyeing med/dark brown hair that is more than 50% gray doesn’t look good to me. Plus I’m not about to do 2 week touch ups. So at that point, I will talk to my colorist about something different...perhaps keeping some gray and doing intermittent low lights?