Jom, according to the Epitaph on John Williams he died in 1887 aged 65 so this would mean that he was born C1822. Rowen was his birthplace and this is in the Parish of Caerhun so when I was in the Archives today I had a look at the Caerhun Baptism records from 1813 - 1870.
Unfortunately they did not have any records that would fit John Williams' profile from 1820 to 1826 by which time he should have been baptised. I decided then to work backwards and found the following information that may be just a coincidence but I'm posting it just in case.
At pg12 No 89 the following details were shown for the Baptism on 2nd Feb 1817:-
John Williams son of Thomas and Emma Williams Abode Rowen Husband's Trade Shoemaker
I carried on looking back and came across this entry:-
At pg 5 this Baptism occured on 23rd October 1814:-
Mary Williams daughter of Thomas and Emma Williams Abode Rowen Husband's trade Shoemaker
Rowen is only a small village today but it would have been even smaller in those days, so is it just a coincidence that there were two Thomas Williams shoemaker in the village or is this John Williams your relative and the dates are wrong?