In the Netherlands, we do group by intelligence, got a whole group of systems for kids age 12 and up. Seems to work well. Still grouped by age too, though you

In the Netherlands, we do group by intelligence, got a whole group of systems for kids age 12 and up. Seems to work well. Still grouped by age too, though you can move up or down a class if that suits you. Grouping by age doesn't always work, as age is a big factor in what kind of people you feel comfortable around.