Drugs...or something else?

This is my contribution to the Barry Bonds discussion, and spins off something Malcolm Gladwell posted in his blog. The home run data I have posted below are for a real player. The age 31 to age 34 stuff might just be a normal age-related performance decline, someone who has been an outstanding power hitter slowly declining to merely above-average. But beginning at age 35, something odd happens--there's a power surge, which continues through age 39, with his career peaks in homers per 600 plate appearances coming when he was 37 and 39. His walks also spiked, rising from about 60 per 600 PA at ages 31-34 to about 90 per 600 PA from ages 37-39. So how would we explain this? Any takers?

Age.....HR/600PA
31.............30
32.............39
33.............35
34.............25
35.............42
36.............39
37.............50
38.............38
39.............51

In case you're curious, this was Hank Aaron...