One of the adjacent parishes is Payhembury, and I am assuming that John and Elizabeth were buried there rather than in Broadhembury, having found likely matches. The burial register entry for John in 1736 stated he is from Broadhembury.
Annoyingly, no age is given for either of them which makes looking for their baptisms tricky. However, I haven't come across any minors marrying in my tree in that era, so I am thinking that would be unusual (esp. for working class folk).
Furthermore, looking at the Clyst Hydon marriage entry, there is no mention of that John & Elizabeth being from another parish but the writer mentions when either the bride or groom or both come from elsewhere on other entries on the same page.
Having said that, though, I am not sure how you know that they might have been minors when they married?
The above is just my observations and unfortunately does not provide any concrete evidence.