I found it interesting that Estonia did so well and yet they don't start until 7, preferring to concentrate on nursery/play based stuff before that getting the children socially ready for school and they also don't differentiate by ability.
The age of starting what is technically described as "school" may be seven years old, but Estonian kindergartens appear to be actively teaching the kids--either that, or Estonia is the kinds of country where you can rely on parents to consistently teach stuff at home. Take a look at this English translation of the academic expectations required for kids to graduate kindergarten:
www.hm.ee/sites/default/files/estonian_national_curriculum_for_preschool_child_care_institutions.pdf
This is an example of the content that Estonian kids are supposed to have acquired as a minimum when graduating kindergarten:
"8) pronounces correctly all the sounds of his or her mother tongue in his or her speech and while repeating the dictated words;
9) knows the letters and spells 1-2-syllable words, recognises some words in spelling;
10) writes 1-2-syllable words correctly in single successive capital letters in the right order; 11) knows poems and songs in his or her mother tongue by heart."
Bear in mind that children have to have completed stuff like the above as a minimum in order to graduate kindergarten and move onto first grade, because Estonia does not do "social promotion" and children can only graduate to the next grade in school if they are considered sufficiently accomplished and have mastered the material they are supposed to have. And that also has to be taken into account when talking about the mixed-ability teaching, by the way--it is a lot easier to teach everyone at one level, when you have ensured a certain level of homogeneity because slow learners get "held back."
Be very wary of "magic country where kids don't learn to read till they are seven!!" With the exception of Denmark, these are all countries which have highly transparent writing systems (i.e., there is a super easy one-letter-to-one-sound rule and only one way of spelling each sound, so learning to read and write is a doddle). In countries like these, children "pick up" reading and writing before school, simply by sharing picture books with parents, getting a few pointers from parents, watching other children, and reading the subtitles on the telly and filling in the gaps themselves. The few that can't read on starting school will find it easy to catch up in a few months, and teachers have lots of time and and attention to give to them because the other kids are reading already. The whole class is then ready, at age 7, to start reading textbooks, writing reports, doing word problems in maths etc. Finland is one such country--Estonia is another, because they undertook spelling reform in the 17th century and have this lovely logical spelling system. Italy, ditto (Italy's education outcomes are a bit rubbish, though).
Denmark is the only country I know of that has a crummy, awkward spelling system (like English but not quite as awful) and still decides to delay teaching reading until 7. Sadly, the delay doesn't turn Danish kids into magic Finnish/Estonian kids--it just means that they are still awkwardly stumbling through their reading until a relatively late age.