Well into their 30s? Not many. Though there have been examples and I have listed them in response to AK who asked the same thing. You can feel free to search out the responses. Should you choose not to, I will summarize and tell you that on a 6 year deal, Kreider will be just fine for at least 4 of the years. And if he spends the last one or two as a 3rd line player that is just fine.
Now tell me other centers like ZBad that have kept being just as productive well into their 30s? Also tell me specifically which if Mika's skills will not be diminished with age?
The biggest trait that players lose with age is their speed. Take Kreiders speed away from him, your not left with much. Take Zbads speed away from him, you still have his vision, passing ability, his shot, his hands. Guys like Stamkos, Kopitar, Malkin are similar players stylistically as Zbad. They are in their 30s and doing fine