More context:
I've been doing a little digging into my Dad's family history, and I'm pretty sure I have located my great-great-grandparents' entry on the Census of Ireland, 1901 as the names match what we have for other documents and the relative ages are correct (i.e. oldest to youngest seems in order).
However, it gets weird real quick: all of their children (that I have thus far found birth or baptism records for [edit: sourced from digitised church and civic records available online]) seem to be between 3 and 5 years older in reality than the ages given on the census. What I've found so far:
Why would they do this?? My family is intrigued, but we're all Australians and none of us are even amateur historians. Our extremely uninformed questions thus far are:
Was it just a family record-keeping error? Was it usual for ages to be a bit loosey-goosey in early C20 Ireland? Was there something going on that would have made it expedient for only one son to be thought "adult"? Would there be some benefit to having younger children (support or welfare)? Were they fae, and disguising their slower ageing?
Some additional facts, if context helps:
I'm still hunting down records for the youngest (age given as 6 on the census), and a brother listed as 16 alongside Catherine. I'll update with their details if/when I find them if that would be helpful.
Ages in the 1901 and 1911 Census records are often exaggerated or understated depending on what state benefit the family might be trying to exploit. For example in this research paper, the author claims that ages for older men were largely exaggerated in 1911 in order to qualify for the old age pension which was started in 1909source .
For your family, it's not certain, but declaring two under age children would probably have qualified the family for child benefits that wouldn't have been available for young adults.
For more a detailed discussion of age misrepresentation see:
Budd, John W.; Guinnane, Timothy. 1992. “Intentional age-misreporting, age-heaping, and the 1908 Old Age Pensions Act in Ireland,” Population Studies, 45(3): 497-518.