How old is a baby when they have their avsd repair?

Depends. There is a lot of variability in size with a VSD and treatment varies.Some will actually close on their own, so nothing beyond watchful waiting is needed.The big ones are monitored and sometimes need meds to support the heart until baby is big enough to make the surgery a bit easier technically.
6 months of age. Or sooner if severe heart failure is diagnosed despite medical treatment. Some advocate earlier repair with no clear benefit.