Snip
In short, I goofed the ages, but I stand by the choice to give the age rather than the event. If your elementary school graduation was so gosh darn important tell me who spoke at it.
Least important question first: my elementary school didn't have graduation speakers. But I do remember both sets of grandparents attending, so it was at least that important to me. But I digress, because translation should never be about me and my personal sentiments.
I'm reading some hostility in your tone and I meant for this just to be constructive criticism (not adversarial), so: polite mode on. As much as I respect your logic and choice to stand by your original translation, I think the scope of your rationale is too limited.
I do hear you about the clunkiness and pain of typesetting the longer "graduation" phrase. But as previously noted, the specific phrase gives us
social context to the relationship between the MC and sensei. By removing that social context, there is now introduced ambiguity about the relationship that (likely) runs contrary to the author's original intent. It's a little bit like a radio edit where the song goes silent instead of introducing an appropriate replacement lyric.
Consider the two phrases side by side, divorced from the setup:
"Oh, I last saw you when you were 10!"
So...how did the speaker know the MC? Baseball coach? Church? Parent across the street? Mailman? Corner store employee? All are equally likely. I have no clue how they will treat each other.
"Oh, I haven't seen you since you graduated from elementary school!"
Ah, the speaker is most likely a teacher/staff who knew MC at that school, or otherwise
knew them as a student. Now I have a baseline for understanding their relationship and personal dynamic.
Important for this story? Unlikely. Important for fidelity? Absolutely.
TL;DR Age is just a number, but social context is so much more than that.