Well, I'm still just about young enough to have done (hypothetically) have a youngster and not be completely au fait yet with the numbering system (and that leaves aside foster, adoptive and grandparents etc. who may be older again) and I absolutely did not go through school with the current England & Wales numbering system even though I was educated in the state sector. We had 2 x R, 1, 2, 3 (actually called Class 1 to Class 6) in the Infants (no I don't quite know how it was organised, I was an infant!), and then 2 x Class 1, 2, 3, 4 (with teacher's initials as suffix) in the Middle School. At High School (which we called it, we didn't really say secondary school in usual conversation) it was 2nd year through to 5th year plus Lower 6th and Upper 6th (no 1st year secondary, in order to keep us in line with neighbouring authorities that went up from Middle School a year earlier). So based on that, I wouldn't have the foggiest.
(Actually I am familiar with the R-Y13 system, but I'm disagreeing with the PP who said surely you came through the system yourself or something similar.)
I think it was a reasonable request, the OP wasn't making a demand but just asking and explaining why. We get a lot of great contributions from across the 4 'home nations' and internationally, and I think that adding the age or approx age in brackets would be useful where school year is relevant, or the main topic, or just age when it's not really.
One thing I find a little confusing is when people write, say, DD3 (for their their third daughter - or is that their third child?) but then other posters use DD3 for their daughter aged 3. Usually you can get it from the context eventually but it can make things harder to understand and reply to initially. But maybe that's just me? 