Justin Morneau and Joe Mauer were already friends when the realization struck. Morneau was in the on-deck circle in Fort Myers, Fla., in March 2002, enjoying the thrill of his first big-league training camp, when he witnessed a moment that, even then, struck him as memorable.

"I was standing there for Joe's first at-bat in his first spring training," Morneau said. "He went up there, took a pitch like he does, then got a hit up the middle, like you see all the time. He got a standing ovation — in spring training.

"I thought, 'Oh, great. I get to follow this my whole career.' "

It's actually turned out that way, mostly, through a career's worth of milestones they've shared — the MVP awards, the fixed-for-life contracts, the weddings, the births (in Mauer's case, the upcoming birth of twins). And on Friday, another big one.

Joe Mauer turns 30.

"It's kind of crazy to think about," said the St. Paul native, a Minnesota celebrity — all-conference basketball player, a Florida State recruit as a quarterback and the first pick in the major league draft in 2001 — for more than half his life. "I don't want to make it too crazy, but it's a birthday. You're supposed to celebrate."

He will party in Chicago, with some close friends and family that he has flown in for the occasion, and for the Twins' series with the White Sox. And maybe he will reflect a little on the mile marker, as Morneau, who reached the round number in May 2011, did as well.

"A lot of good memories, a lot of good times," the Twins first baseman said. "It's pretty amazing how fast it's gone by."

It's official adulthood, basically, no turning back now — but it's hardly the end. In fact, Morneau said he viewed his 30th as a way to look forward, not back.

"You hope it's the start of the second half of your career," said Morneau, who has been a full-time major leaguer since 2004. "You know, in everyday life, with a regular job, when you turn 30 you're hopefully working your way up to a goal, or you're just getting started out on your career. In baseball, if you're like Joe, he came up early, he's been here a while — you're not old, but you're experienced. Your career is ahead of you, but a lot of it is behind you, too."

Past the peak?

How much more is ahead? The statistical peak of the average major league player was once pegged at around 27, but it has gradually grown to a year or later, sabermetricians say. Mauer, who has played 1,078 games before the age of 30 — 855 of them behind the plate — could easily be expected to play a similar number after his birthday, especially if he eventually moves to a less demanding position.

"For every guy who starts to decline at 30, there are some who keep going for a lot longer," Twins closer Glen Perkins said. "He's like a computer with his work ethic, and he hasn't been reprogrammed. There are exceptions to every rule, but Joe is the exception to a lot of rules already. I expect he'll be an exception to this one, too."

That disciplined approach to his work and his preparation might be a big reason why.

"Seeing all the preparation he does, it's so impressive. The games are only a fraction of the work he's putting in," said lefthander Scott Diamond, who does a thorough review of each opposition hitter with Mauer before every start. "Having Joe as a catcher, someone who knows these hitters inside and out, he's great at establishing a game plan and giving you confidence."

Sports are fun, Mauer said, but they're a profession for him, and he made it clear he takes that responsibility seriously. He has developed routines that he rarely deviates from, designed to keep him productive well beyond 30.

"At 25, you work hard on your swing, and taking ground balls. You work on your game," Morneau said. "At 30, you work on your body."

There have been a few well-known glitches in that department, a kidney ailment that cost him a month in 2009, leg injuries that ruined 2011. But "he's holding up pretty well," said Perkins, a fellow Minnesotan who met Mauer when both were 15. "You look at that picture of us as teenagers, he still looks pretty much the same. He's a little bigger, a little thicker, but not much."

And a little more mature, perhaps, his manager said. "He's always been very professional. That hasn't changed a bit," said Ron Gardenhire, the only manager Mauer has ever had in the big leagues. "He probably speaks a little more to the younger guys than he did when he first came up. That's probably the biggest thing you notice — he's a little more open now."

Especially with his family, which has begun growing larger. He married his high school classmate, Maddie Bisanz, in December, and the couple is expecting twins this fall. That might be the achievement that Mauer at 30 is most proud of, more than his three American League batting titles, the 2009 MVP award, the three Gold Gloves and five All-Star selections. More than owning the second-highest career average (.324) of any active player, and the 44th best in history.

"Getting married, that was big. Family is so important in this game to keep you going in the bad times, and help you enjoy the good," Morneau said. "He really enjoys that. It helps him."

And helps him celebrate turning 30, too. "I've never been one to go all-out [for a birthday]," Mauer said, "but this one's going to be fun."