Stephen Strasburg plopped into a chair and faced his locker, one of the double-wide stalls reserved for veterans in the Washington Nationals’ clubhouse, the kind he would have preferred to let someone else have. He glanced at his hand and counted his fingers as he extended them, simple math he needed to prove how long it had been: 2010, 2011, 2012. . . . His eyes widened, and he nodded. It was hard to believe, but it was true: Strasburg had just started his fifth spring training. “That is kind of strange,” pitching coach Steve McCatty said. “It’s weird.” “Time flies,” Strasburg said, a sentiment his friends in the room understood well. During their ascension from doormat to contender, the Nationals have been driven and defined by a youthful, dominant pitching staff. The staff remains excellent, but it is not quite so young anymore. Strasburg, Jordan Zimmermann, Gio Gonzalez, Drew Storen, Tyler Clippard, Ross Detwiler, Craig Stammen — they have all left their early 20s behind. They have attended each other’s weddings. Two of them, Strasburg and Zimmermann, became fathers this winter. They have celebrated each other’s awards and raises. They have supported each other through failures and demotions. As they have helped transform the Nationals, they have grown up together. “I told Clip, ‘I still can’t wrap my head around Stras talking about his kid,’ ” Storen said. “That’s unbelievable to me. It’s cool. You’re able to see people grow up, and at the same time you realize how much you’ve grown up yourself.” The Nationals’ familiar pitchers have treaded carefully into a sort of baseball adolescence. They can no longer consider themselves rookies or inexperienced, but they are not ready to call themselves old hands, either. Gonzalez, Clippard and Detwiler may be closer to 30 than 25, but they haven’t gotten there yet. “I wouldn’t say we’re veterans,” Zimmermann said. “But we’ve been up here for a few years and kind of know what goes on. It’s definitely nice coming to spring training. You feel a lot more comfortable, not like the new guy sitting in the corner, you know?” Last year, Strasburg arrived early for spring training and found an extra-large locker awaiting him. No one had moved into the smaller one next to it, so he told clubhouse manager Mike Wallace to give his to Dan Haren. This year, Strasburg wanted to give his wide locker to Doug Fister, but he arrived too late — Fister’s stuff was already moved in.