I do not think there is such a thing as the “right age gap”. I believe that each family has its own “right gap”. There are two theories on this matter:
1. Have kids close together so they can grow together and be as close as possible socially. There is also a bit of “and let’s get the kids thing over with as quickly as we can and get on with our life” in this approach.
2. Have your children spaced out to allow them to be individuals and give yourself enough time to dedicate to each one of them.