Although there is this view that many countries don't start school til 6 or 7, in my experience it's not quite true, and the vast majority start full time education at a much younger age. It may not be called school but it is still a rich, valuable learning environment.
I know that some countries that 'start school late' do in fact have a school-like pre-school system, but I'm not sure about the 'vast majority'. Finland, Norway, Germany, Switzerland, Austria, ... actually do start late. France has a very school like pre-school education system. So regarding only the ones I know about, the 'vast majority' goes the other way...
In my home country, children start school at age 6 or 7 (when they are deemed, by their parents, to be 'school ready' rather than strictly by age).
To be school ready, children need to by physically appropriately developed (size, stamina, strength, balance etc); socially and emotionally mature enough (able to navigate and behave in groups, emotionally stable and competent, a certain 'work attitude') and have the 'intellectual' preconditions for the first year of school learning. The latter comprise:
- showing interest and curiosity for 'academic' stuff
- being able to visually distinguish symbols (e.g. see that b is a different symbol than d)
- being able to copy symbols (e.g. letters, numbers)
- being able to sort objects or situations according to criteria (big/small, round/corners, fast/slow, long/short)
- being able to compare amounts (more/less)
- being able to recite numbers forwards and backwards (e.g. 5-4-3-2-1-blast off)
- being able to observe and copy motor activities e.g. in craft activities and painting
- being able to apply language with correct (albeit simple) grammar
- being securely able to auditorily distinguish sounds (e.g. b/p g/k c/t)
- being able to copy/recite simple sentences/number sequences
- being able to lay out in language, in a logically and temporally consistent way, current happenings, past experiences and future plans
- able to concentrate, stamina, frustration tolerance
So these are the things that pre-school education (age 5-7) focuses on. If a child is deemed to only partially fulfil these criteria, which is a fair number of children, they join a 'starting' class where the curriculum for the first year of school is split across two years.
Note that children who start school, even if they are 7 already (my DS was sitting his KS1 SATS before he would have started school back 'home') are not expected to even be able to recognise their name or count objects. Though many do, of course, and a small minority will be able to read already.
Pre-school education is also far from full time. Usually it is something like 12-15 hours for 5-6 year olds, and 18-20 hours for 6-7 year olds.
Mind you, primary school children won't have 'full time' school either (e.g. three afternoons off, with school in the afternoons only on Tuesdays and Thursdays, is typical).
I totally agree that the pre-school offer is 'still a rich, valuable learning environment' but it is not just 'not called school', it also looks nothing like school as we know it. Not in how it is structured, and not in what children are meant to learn, academically. But the expectations for the children are a lot more age/developmentally appropriate IMO. And it is amazing how fast the children do learn to read, write, and do arithmetic once they do start school. IMO because their pre-school experience prepared them for academic learning in a very good way, rather than trying to get a head start by introducing academic learning early.
So children who switch systems will be missing the head start, but will have the advantage of being very well prepared for the learning, which is why they tend to catch up quickly.