I wonder if the Scottish system just shifts the problem elsewhere...
In Scotland the official cut-off is 1st March, so with no flexibility on entry the children born in Jan/Feb would be the youngest and therefore disadvantaged by being up to 12 months younger than their peers. However, because they are given the chance to defer with no penalty, this solves the problem for them (as long as their parents make the right choice), so the problem then might shift to the December borns. For them, they are still up to 12 months younger than the eldest in their year (more than 12 months actually, as a few Sep-Dec children have deferred too) and although there are some children up to two months younger than them, these have (in the main) been selected for 'school readiness' as they have chosen not to defer and so are unlikely to be in the struggling group.
I do wonder if anyone has studied the long term effects of December borns in the Scottish system? The other big losers in that system will of course be the Jan/Feb borns whose parents do not choose to defer, but who would have benefitted from it. Now for some children it is probably very apparent that deferral is a good idea, but I suspect that in other cases it isn't clear at all at the time when the choice has to be made. The children who end up not being deferred who then go on to experience problems have their troubles exacerbated by being up to 14 months (or more) younger than their peers and not just 12. It would be interesting to see some studies of educational outcomes for Jan/Feb borns in the scottish system and differences between the deferred & non-deferred subsets.
I actually think a better overall system would be to have 6 month cohorts instead of 12. All schools could have intakes every 6 months. Most A levels and GCSEs can be taken in January or June so the different cohorts could finish school at different times.
More popular degree courses could start every 6 months with less popular ones starting annually (but some of these in September and some in February/March). Gap years could flexibly become 6 months if desired - or a 6 month gap could even become standard to facilitate having results before you apply.
Small schools would have to teach mixed cohort groups, but these would be regrouped each 6 months as the new groups arrived, so no-one would be consistently the youngest.