In my tree on Ancestry I have WD as 1780-1850, but originally had 1791 as birth which made him 11 at his wedding - same conclusion as you. I do not think 1780-1850 are correct either.
WD had a son, William Henry Downey, who was an engraver also. In 1802 WD was an engraver from the Crediton parish as per the Marriage certificate and then in London from 1805 trading as Merrifield & Downey. The Merrifield here was a Thomas Merrifield also from Crediton, who died in about 1804, but Charlotte Merrifield, his wife, per the London Gazette took over. It is possible that WD worked for Merrifield before 1802 either in Crediton or London.
All my ancestors on the paternal side were engravers going back 5 generations. I am almost embarrassed as I cannot draw and ended up as an accountant! My great aunt said that the family originated in Northern Ireland, but so far I have found no evidence.
Does the DFHS have all these BDM records in Crediton?