The youngest I've had is 8 months.

I feel as though my ratios are good enough that I can handle the occasional infant and I don't have a set rule against taking a younger infant given the right circumstances. I think that sometimes parents have to go back to work sooner, and those babies are hard to place in a center. Most centers around here seem to start at 18 months, so these parents rely on home daycare.

I also appreciate that with a younger baby I have the control of introducing the daytime routines early so that transitioning into toddlerhood/preschool years are easier, IMO.