I agree with you. The problem is all kids progress at different rates. Should someone that's ready to start school at 5 have to wait two years to do so? Should someone who's NOT ready to start school at 7 be allowed to wait?
No, I don't think we have to hold the advanced kids back,
or exclude the kids who aren't ready for academics, but could benefit from school in other ways. Though it's sometimes an unpopular opinion in education today, I think we should go back to sorting classes by ability.
We used to do that, starting much younger than we do now, but the problem was that a few kids got sorted incorrectly, put on a lower "track" than they needed to be, and they missed out on opportunities. But instead of fixing
that specific problem by coming up with a way to correct the individual mistakes, we eliminated the whole system as flawed. However, I think is was the
perceived problem, that different classes were somehow "unfair", that actually bothered people, so they scrapped common sense and made it
look fair. But "fair" isn't blindly giving everybody the same thing - fair is doing the best you can to give everyone what they need, without taking it away from someone else.
So I think it's OK to enroll at 5, but group them by where
they need to start. One class might be reading already, one might be learning letters and numbers, and another might not even start that right away. (The biggest challenge I see will be to get
parents to accept that it's really OK for their child not to be in the fastest class.) If children are given
appropriate challenges, they'll meet them - and school will be a place of discovery, not discouragement. And they'll get there, on their own time.