Very few people are actually lactose intolerant (incapable of producing lactase) although any of us can BECOME lactose intolerant...all we have to do is to take it out of our diet, and our bodies stop producing lactase. Takes a week or two of being miserable to build it back up. Ugh. (Sorry, personal experience there...I couldn't look a glass of milk in the face for nearly 2 years after nursing. But boy, do I love chocolate milk & hot chocolate.)
In any event, lactose intolerance doesn't run in my family, so we started mixing up my son's formula with milk (when i lost mine) when he was 9 months. Mixing it in gradually 1:4 for the first week, 1:3 for the second week, 1:2 3rd week until going straight was what our ped recommended.
And count your blessings! :) Nurse for as long as you're able. It's such a fantastic experience combined with perfect nutrition and immune support.
Good luck