Normally, parents always worry about their child; being anti-social or not. There isn't an exact time or age, but usually it's when their child is an adolescent. They will begin to notice how their child is isolating him or herself from activities and keeping their feelings to themselves - rarely sharing what is happening with them or what they did at school.
Everyone develops social skills and being social at different times and ages. They will learn to become social. A quicker way, however, is to encourage your child to join sports or clubs so they can interact with people more willingly.
Being anti-social is not a serious issue. Majority of the people in this world are anti-social, one way or the other. It's important to help them from becoming completely isolated, but a few qualities of anti-social behavior isn't bad. Just make sure you help them interact with other students and they should be good to go.