Except that your children are >1460 days old and >2190 days old.
You count age from 0 in most English-speaking countries (there are places where "1 year old" means 0<=x<365 days old). When you are less than 1, we count with smaller units.
So, my N=3 sample (you and your two children) says we use 0-based counting.
Round(age)? So, they became one year old at 180 days? I doubt that.
And, as I pointed out, there are folks in the world who call their newborn "one year old". And they consider themselves quite sane, thanks.
They count from 1. We count from zero.
Back to the original issue ...
The problem you now have in programming is that it isn't enough that basing from 1 is equivalent. The vast majority of the programming world has converged on 0-based. 1-based has to demonstrate that it is better in order to compensate for the friction of existing in a 0-based world.
And 1-based simply isn't demonstrably better even if I concede that it isn't worse (and I do not concede that).
You count age from 0 in most English-speaking countries (there are places where "1 year old" means 0<=x<365 days old). When you are less than 1, we count with smaller units.
So, my N=3 sample (you and your two children) says we use 0-based counting.
Thank you for making my point. :)