Hi Jonathan, Adult baptism has always been around although its probably more common in recent years when people who have not been baptised as infants decide to join a church.
Also in past years many parents had their children baptised in "batches" to save money (ie would have say 3 children baptised at a time so the eldest one may be 4 or 5 say) although I would think that this doesn't apply to your case.
I suspect he wasn't baptised as an infant & decided to join a church & baptism was required before he could do so.
Good hunting, John
It could also be that he had to be baptised before the church would marry him. This was common even up to the 1960. A friend of mine was married in 1965 and the vicar insisted she was baptised first, although she had been a Sunday school teacher for a number of years she had never been christened.