How old must one be to be considered a 'full price adult' when buying a Seattle Mariners ticket?

My dad and I were planning to watch a Seattle Mariners vs. New York Yankees game on July 10th, but as we were looking to buy tickets, we didn't know whether to buy 2 full price adult tickets or 1 since I am only 12. Please help! This would be our first time watching a baseball game here in the States anyways and we'd like to have the chance to actually watch one this summer. Thanks in advance.