so many players mature and decline at different times. bottom line: it is impossible to say with any certainty when a player will peak or decline. if we could do that, i'd follow the path that Pete Rose tread
That is true, within reason. Your body doesn't naturally decline substantially enough for that kind of drop off at age 29. If a 19-year old kid is having a bad year we can't speculate that maybe his body just declined earlier than most. Something else happened. Yes he's a big guy, but at 29 NFL running backs aren't declining yet and they take arguably the biggest full-body beatings in pro sports.
Yes there are guys that just "lose it" so it's not a given that he juiced by any means, but age isn't a part of this discussion unless there's some rare medical issue or he introduced something foreign into his body. He went from stud to crap instantly and aging just doesn't happen like that at age 29. Ever. Even before medicine was as good as it is now.