I was shocked as an 8th grader that Ben Johnson had tested positive in Seoul in 1988. With so many other scandals and discoveries I have come to believe that they are all using. Golfers use. Ball players use. Football players use. I believe almost all professional (and many many amateur) athletes use.
Humans did not evolve this much since the 70s-80s. It isn't scientifically possible for a human like Usain to run sub 9.7s. Or OLs in Div II to be larger than NFL OLs from 25 years ago. Or 265 lbs LBs to run 4.4 forties.
Those who want to believe in Santa Claus, the Easter Bunny, and "clean" sports are free to do so.
Brady's game is not based on athleticism, but on intelligence, decision-making, and accuracy. There's no reason why he can't still be great at those three things as he ages. The key is that his body tends to not get beat up like other QBs throughout history.
Consider the simple metric of sacks (we know QBs get hit a lot more than just sacks, but still) plus rushes. Remember that years ago there was no QB sliding rule to protect them, like there is now.
- Elway, through age 38: 516 sacks, 774 rushes = 1290 hits
- Montana, through age 38: 313 sacks, 457 rushes = 770 hits
- Tarkenton, through age 38: 274 sacks, 675 rushes = 949 hits (but his sack stat only began in his age 29 year, 9th in the league, so pro-rate those 274 sacks over another 9 years and we're talking about a total of 1194 hits)
- Favre, through age 41: 525 sacks, 602 rushes = 1127 hits
- Marino, through age 38: 270 sacks, 301 rushes = 571 hits
- Fouts, through age 36: 319 sacks, 224 rushes = 543 hits
- PManning, through age 39: 295 sacks, 427 rushes = 722 hits
Brady, through age 38: 370 sacks, 478 rushes = 848 hits
So he has been hit a lot less than Elway or Tarkenton or Favre. More than Peyton, but then again, Peyton's issues stem from his neck injury. Without that, he'd still be throwing darts most likely. Marino and Fouts got hit less, but they still played well late into their careers. Montana got hit less, but he suffered some horrific hits, like the one from Jim Burt of the NY Giants during the playoffs. And Montana was a lot smaller than Brady. Brady is 6'4", 225 lbs. Montana was 6'2", and barely 200 lbs. Marino also suffered a bad knee injury that limited him late in his career.
Brady is just pretty durable, and has taken less abuse over the years than most QBs, especially on a per-game basis.