I'm not sure if I'm supposed to be put down by your comparing me to your teen? I'm not.
Look, I don't know anyone that makes too big a deal of exact dates for th first year because at my age many new relationships among my friends have had exactly the same kind of fuzzy start.
If it goes somewhere, do you count it from the first meet, the exclusivity talk, or from the first email, if you talked for 2 weeks before?
If you want to have some kind of anniversary occasion then decide what makes sense to you - and tell him.
I wouldn't use your approach for two reasons.
Firstly, you want to mark it - if you want to, why are you waiting to see if he mentions it? Don't be passive. Tell him you'd like to.
And if you do want to sit back and wait for him... I think it's a bad thing to decide if he says one date, you'll say - no, later after we were exclusive. Even though that makes more sense to me in a sensible discussion, honestly I'd be pissed off if I proposed something nice, saying I saw it as something to be celebrated, only for my boyfriend to say "no, not counting that".
But mostly, I don't see the point in setting yourself up for disappointment if he is like me - a perfectly lovely committed partner, but one who sees marking longevity for its own sake as pretty pointless.