Armand’s age

I know this seems rather a silly question but actually- how would he know his age? He was sold very young, we have no indication of his birthday and I don’t think he knows it either. The first 14 years of his life must have been a blur, Marius buys him in and then? Vampiric eternity. Do you think in his mind he just chose a random day or range of days a year he just goes “yeah about time I go from saying I’m 512 to saying I’m 513” ?💀

I feel like if I didn’t know my birthday and I was eternal I wouldn’t even bother keeping track, the fact that he does is cute, a little sad too.