It seems hard to believe now but it must of been very common not to know how old you were. I've even seen census where nobody in the family seems to know when they were born. Perhaps their civil birth record got lost I don't know. I don't think birthdays were as big a thing as they are now not for common folk anyway until after WWI. Working 7 days a week in the fields or children in mines working 12 hr. days in dreadful conditions. We just can't imagine
the lives people lived hand to mouth. Children as young as 10 were gainfully employed. I know in the UK in the mines whole families went down the pit even 3 year olds picking coal.
If you look at enough 1901 census you can see how few people could read and write. Less than 50 years before it was illegal to educate an irish child or RC child. They couldn't go to school and attended 'hedge schools' where someone would try to educate children on the sly.
I've seen on census 75 year old women working as servants so I think you worked till you couldn't any longer. No pensions in those days.
You can send for a copy of a death certificate after 1864. There is only the signature of the person reporting the death but the dr. might of signed and written the cause of death. www.groireland.ie