Reason to think Jessi
spoilers below:
Actually in watching the movie, there was a key point or reason to think Jessi wasn't exactly Jessi.
Robert played his 18-year old self and 18-year old Jessi liked him. When Robert confessed to Jessi he was actually 46, sure there were fireworks and bad feelings.
But...
When Jessi wanted to get back with the 46-year old Robert, I knew something was up. It now all makes sense why the 46-year old mother of Jessi WOULD still be interested in the 46-year old Robert.