why can't you give a baby cows milk before 12 months?

I was just wondering what the reason is that babies can't have cows milk before 12 months? my son eats everything milk based rice pudding/yogurt/cheese milk based dishes etc and he loves it!! my mum said when my brother was a baby there was no such thing as follow on milk babies were just on cows milk at 6 months!! so I'm just wondering why they need to stay on follow on milk?