Sunday's Super Bowl 50 — Carolina Panthers versus the Denver Broncos — could mark the end of an era.Peyton Manning's last game.The veteran Broncos quarterback turns 40 next month. After a season plagued by injury and poor play, many suggested it was time to retire. Manning fueled speculation about his future after Denver won the AFC Championship game and microphones heard him tell New England head coach Bill Belichick, "This may be my last rodeo."In the run-up to the Super Bowl, Manning has deflected talk about his career plans, simply saying he's focused on the immediate task ahead – beating the Panthers.And so we wonder - with Peyton Manning, and in a broader sense all elite athletes - when do they know it's time to quit?The short answer – mostly never. At least that's what Leigh Steinberg thinks. He's been one of the most prominent sports agents for more then four decades. His client list includes Hall-of-Fame NFL quarterbacks Steve Young, Troy Aikman, Warren Moon. When asked about
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