I don't know what parents & prospective parents are supposed to do with this info, anyway. If your kid is already born, you can't change their birthday.
Nearly half of pregnancies are unplanned, so oops, you get the birthday month they're given. Of the 60% or so planned, most people would not say "Well it's April already so I'll stop TTC for 8 months until I can be sure baby's birthday will be in the right target zone." Because you just want to get on with TTC, not stop-start every year within a narrow pressurised time window.
Then there's babies born prematurely: you've aimed for an October birthday but the baby comes 6 weeks early in August. Planning gone out the window.
You could have a super-flexible system for starting school age, but only more educated parents would take advantage of it. And then arguably you'd need the ability to move grades or be held back to be made part of the system.
You could have a "test" of school readiness for when to start, but parents would want the final vote to override it anyway, and how would kids with learning disabilities fit in?
You could have an age-adjusted results system, where the results are adjusted by age: but this would be cumbersome to admin to say the least, and would seem silly when they get to age 15-16. There would be no end of arguments about a fair way to enhance results for the youngest pupils at this point.
On the negative side: the eldest get gappy smiles, spots, periods & dental braces first. 