Peyton Manning's best years are behind him. (Creative Commons)

He won four league MVPs — the most ever. He took his team to two Super Bowls, winning one. He had a season, 2004, so flawless, it's unlikely anyone will ever match it.

Among the measurements of the game, passer rating is thought to be the purest form of assessment for a quarterback. Peyton registered a rating of 121.1 that season, the greatest passer rating ever. He also threw 49 touchdown passes, the most ever at the time.

The next year, 2005, he was a bit of a disappointment. His rating was only 104.1.

The next season only 101, and the Super Bowl.

It was something altogether otherworldly and, despite the praise heaped upon him, not truly appreciated for its brilliance.

But it is with a heavy heart I must admit, we have seen the best of Peyton Manning.

You’re a moron, you say. Manning may not have had a great year (by his standards, mind you), but he still broke records, he led the Colts to yet another AFC South title and he did it with an infirmary on the sidelines. He’s still got it, he’s only one year removed from a Super Bowl, for God’s sake!

All this is true. The Colts were decimated by injury this season -- at one time a total of 17 players were nursing injuries. And yes, Manning broke the record for most completions in a season. He also threw the ball 679 times, a byproduct of the injuries. That’s 100 times more than any other season for the legend.

His QB rating dropped to 91.9. For most QBs, this number is enough to retire happily on. Enough to garner 1st Team Pro Bowl honors. Enough to demand big figures.

Not for Peyton. It’s his lowest mark since 2003.

But the most important number is this: 35.

Peyton Manning will be 35 years old next season.

Consider the following:

- At age 36, Dan Marino saw his decline. His quarterback rating dropped to 80.7, his touchdowns plummeted to 17, and only in his rookie year did he produce fewer passing yards in a full season. He lasted two more seasons before retirement.

- Joe Montana's career may as well have ended at 35. His numbers plummeted in 1990 — just a year removed from one of the finest seasons ever put together by a QB (26 TDs and 8 INT in just 13 games with a 112.4 QB rating in 1989).

He played in one final game in San Francisco at age 36 before ending his career in mediocre fashion in Kansas City (two seasons, 29 combined touchdown passes).

- Troy Aikman’s demise occurred so abruptly at age 34 that he retired after tossing just 7 touchdowns against 14 interceptions in 2000. 

- Terry Bradshaw threw in the towel at 35, after two injury plagued seasons.

- Johnny Unitas, whose longevity in the league is well-documented -- he played 17 seasons from 1956 to 1973 -- began to break down at 35. He only played in five games that year and didn’t start a single one. His play never recovered as he sludged through four more mediocre seasons with Baltimore before doing the unthinkable, leaving to sputter out in San Diego.

- Fran Tarkenton's decline came at 37 -- in 1977 he only played in nine games. He ended things the next season.

- Even Steve Young, who in essence got a late start after waiting for Montana to break down, had his last hurrah at age 37 before the concussions got to him.

The examples are endless: Joe Namath (34), Bart Starr (36), Roger Staubach (37), Jim Kelly (36), Otto Graham (34), Dan Fouts (36, dropped off at 35), Sammy Baugh (dropped off at 36 and fizzled until the end at 38), Len Dawson (dropped off at 34), Bob Griese (35) — it goes on and on.

At 34, George Blanda threw 36 touchdowns. At age 35, he threw 42 interceptions, the most ever in a season.

It is the natural course taken by almost all the greats — father time chimes in around age 35 or 36.

Once again, Peyton Manning turns 35 next year, and is coming off his most trying season.

For a stretch of games we saw something, by its very nature, un-Manning. For five games he was simply terrible.

In Week 9, versus the Eagles, he made his 200th consecutive regular season start — something entirely Manning-like. What wasn’t was the interception with six seconds remaining as the Colts vied for field goal position to win the game. They lost.

In Week 10, he threw for 185 yards and no TDs.

In Week 11, again while trying to lead the Colts to a game-winning rally, Peyton was intercepted, his third of the game. Even worse, it came against New England.

In Week 12, he was picked off four times.

In Week 13, another four.

It was inconceivable. For Manning, inexcusable.

In Manning fashion, he bounced back on Thursday Night Football with a huge game.

But the first cracks had shown.

Which begs the question: What will come in 2011? An all-out flood?

There are a few legendary quarterbacks who have had continued success past age 35, but they can be counted on one hand: Warren Moon, John Elway, Kurt Warner (late start) and Brett Favre.

They are the exceptions. It’s unlikely Manning will join that select few.

Nevertheless, we’ll always have those legendary seven years.

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