John Martin Biron, her son received into the church at Topsham,19.9.1821, note says he was baptised at Emsworth. (might be worth checking that one out, to confirm fathers name as John,(per marriage cert of 1845) and fathers occupation. As he went on to be a doctor, there must have been some money in the family) John M Biron died 1870 aged 53 so was born 1816/17, father could have died between 1816 and 1821.

Susan died aged 86, according to G.R.O. so must have been in her 30's when he was born, might even have been older children before him? Or they got her age wrong, as she is down as 71 in 1861 census.
Hope this helps.