We were in the opposite boat... so here are my observations from that side of the fence:
In the school my son went to, he was the ONLY 5yo in K. All the rest were 6 & 7.
Going from a multiage school (2-6) to an equal age school the differences didn't bother my son at all, but they bothered a number of kids who had been in preschools where there were 2yo rooms, 3yo rooms, 4yo rooms, etc. Same token there was a lot of discord in the other classrooms in the upper levels (when 5th grade can have 10-13yos in it there is a HUGE developmental dissonance).
I found it to be really really *strange*, because schools that *intentionally* have multiage classrooms the opposite is true. Younger kids pattern off of the more mature (calm, focused, behaving) older students, and older students go the extra mile to be kind to / help out the younger kids. And regardless of ability those who are more advanced help out those who are less advanced (so a 4yo could be teaching a 6yo how to do a certain work that the 4yo know and the 6yo doesn't as well as vice versa).
I think the problem comes from expectation. In single age classrooms there is an expectation by both the teacher AND the students that "everyone is the same"... and people who know more are nerds and people who know less are stupid... as opposed to people just knowing different things. Or that younger kids are going to be bouncier on average and older students are going to have longer attention spans so one student asking an advanced Q isn't a "nerd" they're just older (usually) and the expectation by the other kids is that when THEY'RE older, they'll have similar Q's. Or that the person just knows more about x than they do, but if they spent as much time on x instead of y they'll be equally good at it.
It's OBVIOUS that a 5yo is going to be less developmentally mature than a 7yo... but when they're both in the same class (in a single age school) the same expectations are placed on both. Creating a lot of frustration on everyone's part. Add in hormones in later years by the older kids and there was real strife. But in multiage classrooms the expectation is that kids are at different levels, so both the teachers and the students are a lot more fluid & accepting of individuals.
If we had to go back into the PS system for any reason, I would fight tooth and nail to get into one of the multiage/montessori programs. The 3 year split between students (5 turning 6, and 6 turning 7 creates classrooms with 5, 6, & 7 yo's in K ... 10, 11,12yos -and sometimes 13 in 5th .... 11, 12, 13 and sometimes 14 yos in 6th). And that's not even including the MUCH older students... there were two 7 turning 8yos in my son's K class, and then there are kids who are held back for academic reason. So it can easily become a 4 year spread. Of course, in middle & highschool students are grouped by ability instead of age... but elementary is super dicey.
In my experience/ observation ages don't really sort themselves out until college, although it can sometimes happen in highschool. Also... that a 1 year difference means very little overall, but a 3-4 year difference (when one finds 5 turning 6, 6 turning 7, and 7 turning 8) creates a HUGE difference.