Recently (within a month or so) my training partner posed the question: Could you still PR at distances you have already ran? My answer: certainly. I am 23 years old and healthy. But I also added the disclaimer that I would have to find the time and motivation that I had as a college student-athlete. It isn't possible to work and run. I once ran 116 miles in a week that I worked 50 hours of hard labor. Now I work 40 hours of fairly easy labor.
It really all goes back to the motivation factor. There isn't a lot of incentive to run fast for the vast majority of runners. Prize money, when available, pales in comparison to the amount of hours of hard work that is put in. Health benefits can be had with general fitness, no need to get out there and run tempo runs, hill workouts, etc. It really all comes down to a sense of self-satisfaction. For now, I am happy being somewhere in between hobby-jogging and a truly competitive runner. I really love the activity of running. Racing is so fun but the amount of work that goes into a succesful race can be taxing mentally and physically as well as on your time. Maybe I just need a little mental break from 7 years of nearly year-round training and competition.
Back to the original question, Can I still PR? Yes, will I? We'll see. Maybe I'll stop running the same events that I used to run. Trail races of all distances are currently the most appealing to me. That is a different type of training, one that appeals to my motivation and time better than track and road races.