Since he is in the news, I thought it would be good to revisit.
I have written about this before and there are some links below. This table shows his neutralized SLG as he aged from Baseball Reference. This means that it was adjusted for the league average and park effects. Notice how his 4 best years are clearly from 36-39. This seems to be highly unusual and I think his improvement at those ages over what he did earlier is much better than anyone else's improvement.
Now links to other posts:
The Best 5-Year Hitting Performances By Age
Bonds Aging vs. Aaron Aging
Bonds Greatest Feat Might Be Improvement
Has Anyone Aged as Well As Barry Bonds?
Has Anyone Aged as Well As Barry Bonds? (yes, there are two differenct articles with the same name)