Age at baptism ? The girl could be months old or 'X' years old when baptised. The (bride) girls birth certificate should give a more accurate figure of her exact age with the known parents and place of birth-mothers maiden name etc)
Age at death ? (My great granny lost 3 year in age at death from her birth certificate year and marriage age) can be out +/- a year depending when born in a year, ie:- born Jan 1st or born Dec 31st in the same year or the date in a year when died. The informant may not know the exact age at death and states to the best of his/her knowledge of the deceased's age at death.
Age at marriage ? (many a yarn told there _ my mum thought dad should have retired on state pension 3 years before he actually did reach legal pension age 65 in the UK
)
Age on census can be +/- a year or two out
These records you mention (apart from a birth certificate with known correct facts of parents of the right birth register exact date of the baby child birth ) are not that accurate as counting in months or even 1 to 2 years.
I don't know about Ireland but in England they would have had to have had a special licence to marry with parent father or male guardian bond giving his permission for the minor to marry.
Age of consent to have intercourse and age to marry may have differed
