Oh! Yeah, that is a problem. I honestly decided Sirius was either talking about Rabastan and Rodolphous (who could've been a bit younger), Snape was friends with the older students, or Sirius was just being bitter. After all, he also said Snape was really popular, when OoTP contradicted that.

Plus, JKR contradicts herself. The tree also makes it look like wizards have perfectly normal life spans, when she said they had longer ones...