What was the exact age of Vanessa in the first season?

Vanessa was 11 years old. In fact, Here are all of the Kids Ages at the start of the show

Denise- 16
Theo- 14
Vanessa- 11
Rudy- 5
Sandra- 20

Those ages are correct. Some sources in books say that Vanessa was eight. Maybe she was meant to be eight but was changed around because she was eleven according to some episodes. No doubt about that.