It all depends on how well trained you were when you were young. I seem to have hit my peak when I was about 38, setting PRs at the mile, 3k, and 10k in the space of a few months. My mile time was about 7 seconds faster than my college PR - but I wasn't very well trained (or coached) in college.
There are masters age grading tables out there that give you an idea how much you should expect to slow down, and they predict that, on average, your mile time (for example) get slower by a couple of seconds a year after age 33 - but your mileage may vary, as they say.
Since I happen to be almost the same age as Regina, and not nearly fast enough to make it worth my while to take drugs, I don't think her ability to set a PR at age 39 proves she's on drugs.