What is the earliest age at which you could begin to insert ear tubes?

As soon as indicated. The usual indications for tubes are 5-6 episodes of ear infections/year in a single year. If a child develops repeated infections in the first year of life... They can have tubes. Newborns with persistent fluid may need tubes to drain the fluid. Bottom line, there is no wrong age... If there is a problem with retained fluid or recurrent infections.. Tubes can be placed.
Depends. Once there is established frequency of ear infections.
Depends. They are routinely inserted in some cleft lip/palate patients with their first surgery(a few months or sooner), since these kids have such a high frequency of middle ear disease. In other kids it depends on many factors and is best left up to those who must make the decision.