I transitioned Emma over to adult food around 11 months of age - she gets 1/4-1/3 cup twice a day of Summit, with green beans added. Liam I put straight on adult food when I got him. He tends to inhale his food, whereas Emma eats a little more slowly.
What I have noticed: If they have set times for their food, they are more apt to eat it all. You have better control over how much they eat, whether they eat it or not. I have left food down for Noah and for Emma before Liam came. They didn't like to eat first thing in the morning but did eat it within a couple of hours of getting it.
I strongly believe in having set times and set amounts of food for dogs. You can monitor their weight, their health, their appetite, whether they have gone off a certain food, etc, rather than free feeding. I am speaking just for me, but I know others feel the same.