He was ten, ten was right on becoming a man, and he knew it.  Eight, nine, you don’t really feel it.  But ten, you start to, and by eleven you are sure.  That is, you are sure you’re becoming a man.
To him it made perfect sense, soon he will be a teenager, 13.  Though he didn’t know then that one doesn’t really feel like a teenager just because one’s age ends with “teen.”  In fact he wouldn’t really feel like a teenager until 16, 13 was a lie to him.  But ten, ten he felt, he felt the full power of two digits.
Later in life he’d find numbers arbitrary and thereby irrelevant to divining true insight into his character.  He’d feel too old or too young, or that he was not appreciative enough of his numeric age to be doing enough with it, by it.  That is, if he really appreciated his youth he’d have done so much more in those years when he was 8 or 9, or 18 or 19, or 28 or 29, or so on.  And he could feel it in his bones that he wasn’t going to do much more when he’d turn another age.
Birthdays, too, were arbitrary numbers.  So the earth revolves around the sun in 365 days, so what?  So what if that’s how we count our years, so what?  Suppose he’s off by one day, suppose his birthday was yesterday, then it would already be over, suppose it’s years from now, when he’ll be dead, no one to celebrate, no one to not celebrate, who cares?  Sadly, this was every birthday since he was ten, since he started becoming a man.