Calvin Johnson had the greatest receiving season in NFL history last year. He had the greatest receiving game in NFL history earlier this year.

He's amid the most prolific three-year stretch by a receiver in this game's history.

He's 28 years old.

Calvin Johnson is in his prime? Not really, according to one Baltimore Ravens defensive back.

Megatron would appear to be at the height of his prime. But that's not so, according to rookie Ravens safety Matt Elam.

"He's pretty old, so I don't know how physical he'll be," Elam told local reporters in Baltimore when asked about covering Johnson. "He's a big guy, but he's older."

Elam's comments about Johnson's physicality are odd, considering Johnson is 6-foot-5 and among the most physical receivers in the NFL. Elam is a 5-foot-10 rookie.

The comments about his age are peculiar as well, with Johnson squarely in his prime. But he does broach a subject that is on the horizon, which is just how long Johnson can remain the game's greatest receiver before Father Time catches up to him.

He hasn't slowed just yet, but is starting to develop chronic knee issues that have limited him to just a practice or two a week for much of the season, and cost him one game.

"I'm not worried about (the knee)," Johnson said last week. "Like I said, when the time comes (to hang it up), y'all will know. It's not here right now."

Johnson can't be this good forever. But for right now, he is this good, and Elam will get a close look at just how good he is soon enough.

Information from the Associated Press was used in this report.