I think we may be just from different semantics on that. For me, the 24th birthday is the start of the 24th year - the year you spend at age 24 might be a better way to phrase it. I can see it your way of course, it's just not from my frame of reference I guess. Like, I had my 37th b-day last month, that day marked the beginning of my 37 years walking this earth. It's almost like not counting the first 12 months of infancy honestly lol (like the counter starts at 0 and you build up to age 1 and so on). It's just the way I know it at least.