agree with you that Jean is a mistranscription, here's the original
http://www.rootschat.com/links/01hwc/
Splendid - thanks Osprey - that's the record I'm familiar with. I love your short URL links to the familysearch scans, by the way.
What source are you using for the ages of William & Jennifer? There's nothing on the marriage to suggest they are under 21
http://www.rootschat.com/links/01hwd/
There would appear to be 2 couples of similar name having children baptised in St Agnes at the same time
William Allen & Jane Martin who married 5 Aug 1763 & William Allein & Joan Dale who married 5 Aug 1775, both couples had a son called William, the older one being son of William & Jane baptised 23 Aug 1766. He may be the William who was buried 17 July 1831 aged 63 of Rinker's Wood.
I think the reason for the early marriage is obvious, married in Jan 1794 and John Paul Allen was born 12 April 1794 according to his baptism.

Yes, I think John Paul's birth in April indicates that William and Jennifer might have been a couple of trimesters closer to being parents on their wedding day! The little scamps.

The point you raise first in the quote block above is exactly my conundrum. There isn't anything to indicate that William and Jennifer are kids on their marriage record. However, when I look at the names of their children, I can't resist having Jennifer (b. 1775) and William (b. 1776) as the parents.
William Allen and Jennifer Paul's first son in 1794, seconds after they tie the knot, is John Paul Allen. Jennifer Paul (b. late 1775) has a father by the name of John Paul.
Second child, and eldest daughter Mary Ann might well be named for Jennifer's mother, Mary Bray.
Third child is William; he's clearly named for the paternal Williams.
Fourth child is Elizabeth. Not sure of the significance of that name - though it's pretty ubiquitous in the 1700s. Both Jennifer (b. 1775) and William (b. 1776) have sisters named Elizabeth.
Fifth child, though, is my Joan Allen. And Joan is the name of the 1776-born William's mother.
William and Jennifer have three daughters, but there is no "Jane" for Jane Martin. Jane Martin seems to have been baptised in St Agnes on 06 December 1740, the daughter of James Martin (he probably witnesses their marriage too). But James isn't commemorated in the names of William and Jennifer's children either. All this makes me less inclined to think that Joan Allen's father William is the 1766-born son of William Allen and Jane Martin.
Regarding this other contemporary Allen family from St Agnes, and just for the record, it seems probable that Jane Allen (nee Martin) is buried in St Agnes in 1810, aged 70. This matches nicely with her 1740 baptism. There's a William Allen, aged 80, buried in St Agnes a year later. Both burial records indicate that they were living in Perranzabuloe. If these burials are Jane and William, who's the other St Agnes William Allen that you've turned up, Osprey - the one from Rinker's Wood who is buried in St Agnes in mid 1831?
I don't know, readers. It really does seem like there's (at least) two William Allen families and either could be the correct ancestral line for Joan Allen (and John Paul, etc.). Am I being too hung up on children's names, and overlooking something obvious, perhaps?
Greatly appreciate all this help!

Grant