Start Students According To Ability Not Age?

Swedish school officials are considering starting students according to ability not age. I have no idea if this is a good idea or bad. The only thing I thought of was the socialization issue. I remember being the youngest in my class and I felt an equal to my class not to my age group. This had advantages and disadvantages. On the plus side I was fearless in sport and interpersonally on the minus I was fearless interpersonally.