This is a new age for the Mets, and yesterday they showed a new ace. Jon Niese earned his No. 1 status in the Mets’ 11-2 thumping of the Padres on Opening Day at Citi Field. It wasn’t just the 6 2/3 innings the lefty pitched, allowing four hits and two runs. It wasn’t the way he insisted on going back out for the seventh inning and it wasn’t the way he accepted his role of replacing Johan Santana. It was this: Jon Niese set the tone that the Mets can grow their own when it comes to starting pitchers and now the torch will be passed to Matt Harvey for Game 2 tomorrow night. The Mets are trying to establish that they can build their own starting staff and in so many ways that is what this 2013 season is all about. Niese is playing the role of Andy Pettitte for the Mets and he did exactly what was needed yesterday. “He has stepped into a role where he kind of leads that rotation,’’ manager Terry Collins said. Coming through the Mets system makes a difference. “At least in my situation, you come through this system and you have a great deal of pride of putting that uniform on,’’ noted David Wright, who has embraced his role as captain. “That is what we are trying to do here.’’ Catcher John Buck, who is going to make a big difference for this pitching staff, said Niese lived up to the billing. “He showed today why he is our No. 1,’’ Buck said. “I’ve said all along he could be a No. 1 on a lot of teams and he showed that. He had a lot of pressure coming in, being moved into that role with Santana being out. His first home opener, he stepped up and did what No. 1 should do. He’s been compared to Pettitte and he didn’t change who he was, he has the guts to stick to who he is. That says a lot for his character and his makeup.’’