DENVER -- Rockies top prospect Nolan Arenado turned 21 at the start of last season at Double-A Tulsa. It turned out he was old enough, all right ... old enough to experience stress in a baseball uniform for the first time.But as often is the case, the rough early days of true adulthood have given Arenado wisdom. Arenado, a power-hitting third baseman and the Rockies' second-round pick in the 2009 First-Year Player Draft, recovered from a rough midseason patch to finish strong for a Tulsa team that made the Texas League playoffs. Heading into his second Major League Spring Training, Arenado reports being in better shape physically, and believes he is better able to put aside the pressures that come with climbing to the big leagues. "Honestly, by the end, I was having the most fun that I'd ever had with my teammates, and I realized you've got to have fun with it," Arenado said while leaving a workout at his offseason home in Lake Forest, Calif. "I had some stressful moments that hurt me as a ballplayer. This year, I'm going to have fun the whole time, play the way I know how." Arenado hit well early in Spring Training last year before tailing off. He started last season ranked as the Rockies' top prospect and one of the best in baseball, and he began the year on fire. Although he began to cool in May, he flirted with a .300 batting average through June. Talk of a promotion to the Majors, with the Rockies struggling, was more than a whisper among fans and on social media. But in late June, general manager Dan O'Dowd said in a conference call with Rockies fans that Arenado's "maturity level still hasn't caught up with his ability level," and all but ruled out a callup. Arenado's .165 July batting average removed any hope of the Rockies changing their minds. The Rockies' assessment of Arenado was frank, but not harsh. They haven't soured on him. But the Rockies have Chris Nelson and Jordan Pacheco coming off high batting average seasons and DJ LeMahieu, also coming off a good year, possessing the ability to play there. Colorado also traded lefty reliever Matt Reynolds to Arizona for third baseman Ryan Wheeler. Arenado will be back in camp as a non-roster invitee with hopes of making the squad, but the Rockies don't have to rush him. "He learned to focus on what's in front of you -- just play the game, and not worry about a lot of things that may or may not come in the future," said Bill Geivett, the Rockies' senior vice president of Major League operations. "He came into camp trying to be on the club and put a lot of pressure on himself, and did throughout the season. "That's part of the maturity of a player. You can't focus on what may come and things that are out of your control. That was a real big growth year for him and his maturity at the professional level, and eventually the Major League level." Arenado led the Minor Leagues in RBIs and earned Arizona Fall League Most Valuable Player honors in 2011. The mental lapses during games and frustrated jogs up the first-base line on ground balls were indications to the Rockies that he wasn't ready for a big league atmosphere, regardless of what his stat sheet may have read at a given time. Arenado admitted that the down times were difficult. "I thought there was a chance of getting called up, and that was part of it," Arenado said. "But it was tough dealing with the slumps and losing road trips. Those weren't easy to deal with at first, and when you're on a bus for 14 hours, it starts to wear on you." From a pure numbers standpoint, few players on any given Minor League team are expected to even make the Majors, much less become stars. A callous outsider view is that the teams are organized because the prized prospects need enough guys to play the games it takes for them to prepare for the Majors. But those players are flesh and blood, they're valuable, and who's to say who won't make it? Arenado learned the value of good teammates who have their own big league dreams, but were cognizant of the task in Tulsa. Arenado finished with a .285 batting average, 12 home runs and 56 RBIs in 134 games in 2012. His confidence comes from hitting .366 (45-for-123) with four home runs and 13 doubles in his final 30 regular-season games. "I had good teammates and good people around me, and they understood that things could get better," Arenado said. "That's what we did. They helped me get my mindset to where it was supposed to be. "It turned out to be a very good year from the respect that I learned a lot. Numbers-wise, it was not what I expected, but that's not what it's all about. I did expect more, but I finished strong and that was my goal. I ended on a good note and helped the team win." For a brief period in December, as he did in 2011, Arenado did part of his training in Las Vegas with Rockies shortstop Troy Tulowitzki and free-agent first baseman Jason Giambi, who was with the Rockies from late 2009 through last season. Arenado could see that he had matured physically. "When I went out there with Tulo and Giambi, I was in better shape than I was last year, and I could definitely hang with what they were doing," Arenado said. He promises to be ready mentally. He plans to arrive at camp a little before pitchers and catchers are due to report on Feb. 10. One variable is he'll be trying to impress new leadership, with Walt Weiss having taken over for Jim Tracy as manager. But he plans to not let his task create unnecessary stress. "If I start to worry about impressing coaches and the manager, I'm going to get myself in trouble," Arenado said. "I plan to show them what I can do, but the key is going out and having fun."
Thomas Harding is a reporter for MLB.com. Read his blog, Hardball in the Rockies, and follow him on Twitter @harding_at_mlb. This story was not subject to the approval of Major League Baseball or its clubs.