I had the same question. In the last story, I had the impression that Lisa and Jeff had only just gotten engaged after being together for a long time. They were getting married two weeks later.
I think the best interpretation is that they've been together for five years and this is just after the last book, and the age for Peter is wrong...but that's just a guess because in the last book Andrew hadn't moved in with Will yet.
Another option is this is five years after the last book when Lisa and Jeff got engaged, and it's a different Peter, but in that case Andrew should've finished school...
I'm going to ignore the continuity issues and just enjoy the story