30's is usually when the decline starts. But as far as PF, this tends to crop up in the 40s, in the general population, and males. Usually it's the extra weight we put on that stresses everything out it seems, but we have people getting back into it @ 40 (masters) too.
In my experience, I got PT @ 34 and it was a gradual decline until 40, when I decided to quit running and take up golf. FF to early 50s, got back into it, got a hammy, a AT, and PF, by late 50s- but was still able to race fairly well after getting the injuries to cool down.
Now @ 60, I did hit my Age grade times in the 5K from back in the day-so far, but seem to pay for it with minor injury flare ups that seem to cycle around my body, LOL! ? This makes it hard to keep "training", but I'm still doing easy miles while I wait for everything to cool down. I would like to think I'm fairly typical of most runners that try and stay competitive throughout their life.
I read a study a few years back where race participation in the 60-64 group does a nose dive, and I see why. The good thing is you have a better chance to take home some hardware at the local 5K, if you can continue to train. :)
So, no hard and fast answer but the 4th decade is probably the decade the real decline starts and continues on into the 5th and on. It's not all doom and gloom, but if you can keep the extra lbs. off (within 5 lbs of your prime weight), and you keep a moderate training program- and stay away from marathon training ;), you will probably last longer.
If you want to do the marathon thing, do it before late 50s. Now ultras might just be easier on a person because of the pace and you are fueling and walking and usually on trails.