Getting older shouldn't effect it... in fact, with great artists it often makes it richer. I feel Em is one of those artists, as his writing in many respects has gotten richer in areas, more adult.
You say 'not much to talk about' but that's not true. If the artist is creative... there's no age limit on creativity. Unless you just want him to talk about his own life constantly. Which to me is boring.
I used to think he would for some reason look ridiculous rapping at 40, just on paper, 'white 40 year old rapping' sounds very silly. But, he's managed to re-define it, if you look at like "Space Bound", it feels very toned and atmospheric, it doesn't feel like 'silly rap', and he has a better look now.
So, I think he can go on as long as he wants but, I think he'll probably make like 2-3 more max.