Have I understood correctly - the age of William Henry is assumed to be the same as Harriet ?
Dodo - you mention William and Harriet's marriage certificate

Do you actually have a copy of that ? It should give William Henry's age at marriage

There is a William Burt age 35 on the 1841, in Plymouth area, a shoe maker, wife Mary - but no younger children

HO107. P. 274/4 Fol. 9 Pg. 13