Greg Jennings believes in his smellf. Just check out his hilarious Old Spice ads.

But do NFL franchises believe in him over the long haul?

If Jennings – Green Bay’s star wideout – were two years younger with a clean medical history, he might be the most sought-after player, at any position, in this year’s free agency class. (Or, more likely, the Packers would not have let him reach the open market to begin with.)

But this isn’t 2011. He’s coming off back-to-back injury-shortened seasons – playing in just eight games (starting five) last year, the fewest of his career.

And he turns 30 in September, which for some receivers, might as well be 50.

Yet he’s in line for one last mega payday. He’s among the top options at his position – and a possible Dolphins target when free agency opens Tuesday.

But there’s a question that should give any general manager pause: Will we get the Jennings of 2010, when he had 12 touchdown catches and more than 1,200 yards receiving? Or the Jennings of 2012, when a sports hernia derailed a season that featured career lows in catches (36) and yards (366)?

“You notice it with age,” said retired Dolphins wideout O.J. McDuffie, now an analyst on 940 WINZ. “The older we get, the harder it is for us to come back from injury.”

McDuffie speaks from experience. He led the NFL in receptions in 1998. But two years later, he was out of the league. McDuffie caught just 22 passes after his 30th birthday.

AGE NOT JUST A NUMBER

Surely, age will factor in as Jeff Ireland mulls whether Jennings, a two-time Pro Bowl selection, deserves a mega, multi-year contract.

“Worth is in the eye of the beholder,” said ESPN’s Bill Polian, a longtime personnel executive in the NFL. “Elite receivers, and I would categorize him as one, tend to begin to turn down about 33 or 34.

“How long a contract do you give him and what is his potential to continue to play at a high level, given the injury and age history? Each individual club has its own metrics that would tell him that.”

There are some in the league, Polian added, that wouldn’t even consider giving a player over 27 a long-term, big-money deal.

Others, however, are more willing to gamble, particularly if they are convinced that they’re one piece away from competing for a championship.

With four straight losing seasons, the Dolphins are probably further off. But wide receiver is their most glaring need.

And Jennings – an affable, fan-friendly star – has something else working in his favor: His most productive seasons came when Joe Philbin was his offensive coordinator.

“I love Joe; I had a great relationship with him,” Jennings told the team’s website during Super Bowl week. “He’s one of my favorite coaches.”

Jennings, a preacher’s son from Kalamazoo, Mich., averaged more than 1,100 yards and nine touchdowns in Philbin’s five seasons leading the Packers offense.

By comparison, the entire Dolphins’ stable of wide receivers managed just three receiving touchdowns last year.

‘SMOOTH ROUTE-RUNNER’

And while he might not have the speed of Mike Wallace, Jennings is still plenty fast, running a 4.4-second 40 coming out of Western Michigan. Plus, he’s probably a better all-around player than Wallace, his main competition for free agency dollars.

“He’s a deep threat, a smooth route-runner,” said former Packers running back Gary Ellerson, who’s now a radio personality in Milwaukee. “Reggie Wayne comes to mind. He’s got exceptional hands and can get deep on anybody. His speed has been underrated a lot.”

“You’ve got the Calvin Johnsons and Larry Fitzgeralds, and then the next group,” Ellerson said. “He falls in the next group of receivers.”

That group is still better than anything the Dolphins had in 2012, and Ellerson believes Jennings would be a good fit in Miami – under the right circumstances.

With Jennings’ age and history, the Dolphins would be well-served to draw up a contract rich with caveats and incentives, Ellerson said.

“At some point, you don’t know if he’s already hit that wall,” he added.

That’ll be up to Ireland to decide. Sometimes it simply comes down to believing in the player. And him believing in his smellf.