First grade is 6 or 7 years. It depends on schools and parents, my class had majority of 6 aged, but my friend had 7 aged majority in his class. If you are 5 ages old but will have 6 in November for example (school starts in September) you can enter the school.

It is supposed that children can don't know how to write and how to read. So studying begins from ABC and letters writing. For example I was able to read, but didn't know how to write. There were children who could write and who couldn't read in my class. To the end of 1st grade children should read 30-50 words per minute and write simple sentences (without commas etc). Not sure about counting, but digits and simple numbers are studied too.

There are also preparatory classes, but they are not state and not obligatory. So it is supposed that children can enter school without any knowledge. General education (1st-9th grades) is obligatory for children here. If parents do not allow children to go to school, they will be forced or children will be taken by the state and ex-parents will be forced to work to pay for children's education.