In Justin’s terms, this episode is all about his mother’s relationship with Tucker and his inability to accept or embrace it. In scene after scene, he is rude, condescending and completely unaccepting. Regardless of the age difference between him and Brian, he is unwilling to accept this same kind of thing from his mother. Is this all there is to it? Or is there a deeper reason for his attitude? Could it be that Justin is projecting his own feelings of dissatisfaction with Brian and the break-up here? Remember that post break-up, he’s been surprisingly calm and accepting. Could there be suppressed feelings that are finding there way out when he has to face his mother’s new relationship?
I think this at the heart of Justin's bratty attitude re: Tucker.
I don't think he cares so much about the fact Tucker's younger than Jennifer, than he does about the fact HE'S unhappy with his own relationship. He's broken up, and probably heartbroken. He's put on a brave face for Deb and Emmett, even Jennifer and Michael, but come on, he must be devastated his relationship with Brian is finally over. And I do believe he thinks it's over for good. Done. The End.
So one way of channeling his feelings is by taking potshots at Tucker. Being a brat. I don't think Justin's such a hypocrite that he thinks the age difference is a genuine factor.
I'm sure there's also an element of "not liking the new guy" whoever he may be, if this is Jen's first post-divorce relationship.
However, once he and Brian reunite, Tucker's a non-issue. He even invites him to the wedding. I think it's Justin's own unhappiness that's at the heart of h brattiness re: Tucker.