In Season 5 Episode 2 "Boy Meets Real World", Eric claims to be 8 months older than Jack but then in Season 5 Episode 17 "And Then There Was Shawn", Jack claims to be the oldest of the group. Of course, Eric also mistakenly says he's the oldest in that episode before Jack corrects him so it's possible this is just another example of Eric being Eric.
In the previous episode "Brothers" Shawn recognizes Jack's face and speaks to him, in this episode he said he had always wondered what Jack looked like.