Ali, hell don't even get me started. It was sad, and painful to watch. While truly the greatest, he could not beat father age in a sport that rarely sees their pros stay past their early 30s,
same as an NFL QB. Ali fought with his old form, arguably, into age 30, but he was 36 at his last fight and 37 when he hung it up. By then, even the hardest core fans knew he was done. It was clear the only person who didn't know he was done was Ali himself.
So what about football and similarly old QBs in a physically brutal sport?
I was in the Bay Area when there was a huge freakout when SF replaced Joe Montana (
at age 34) with Steve Young. Montana had seen his greatest years behind him, not in front of him when SF made the switch. Young brought SF to three NFC games and one Super Bowl and continued the 49ers dynasty started in 1981 into 1999. Only the Cowboys had a better long term run.
Choosing the younger Steve Young was the right move.
Young would follow his '94 SB season with play after that into 1999 and retire the highest rated QB of all time, where his rating still stands, with only Tony Romo rising in quality as a player (these days) on track to catch Young's record. Former rising QBs in spitting distance of Young, who are Manning and Brady got close in earlier times, but have started their overall passing efficiency decline. Overall, Brady had his best season in 2007 and Manning, perhaps the best season of any NFL QB, in 2004.
Based on Montana's play starting into his early 30s, and his pretty decent work late career work with the Kansas City Chiefs, the man could have "technically" played until age 45 and still kept his numbers above the average starting NFL quarterback.
Personally, I would not have liked to have seen Montana play until 43 to 45, with perhaps the last five years with him as a third string "just in case" player and elder mentor. I didn't want Montana to become a man with that name, with the bench time of a 40-something Steve DeBerg, known by many as Steve "da backup".
I read on insane article who claimed that they thought Manning could play into his mid-40s, realizing that his amazing brain is also an asset that could orchestrate younger QBs and help lead the Colts to more postseasons.
OK, I get that logic in a world that is in a parallel universe where there is no such thing as money.

But it's assuming Manning's skill and health stays the same into age 35, 37, 40, 43?!? Yeah, right.
But let's look at this thing fiscally. Let's say Manning plays for ten years like the article I read in my local paper.
1) How long can he be a starter?
2) Will he become more prone in injury from now until ten years from now?
3) Why didn't Aikman (3 rings) or Young (highest rated QB) play to or past age 40?
4) Could your budget afford Peyton Manning for 10 more years, with the second half of his ten years being essentially a uniformed QB coach?