Your generation name only signifies who you took vows under. If you're a De (31st), you took vows under a Su (30th). It doesn't have anything to do with age or seniority. For example, I'm almost the same age the master I took my layman vows under. He's a De so I'm a Xing (32nd). I'm currently training under a former Yan (34th) and he's considerably younger than me.