Originally Posted by Pestle:
After surviving three toddlers with behavioral issues, I'm changing my tune. I didn't want infants, but I'm now of the opinion that I should take them as infants and grow my own toddlers. 
This is so true!
Some of the most difficult kids I've had in care were those that started after age 1-1.5 yrs old. The easiest kids I've had over the years are those I've gotten in the first year of life. Helps that they basically learn within your environment so my rules are all they've known.
I find that separating the kids not so much by age but by ability makes a huge difference. Once my kiddos start walking (full on walking) they are no longer allowed in the non-mobile areas. Once they master toilet use and can communicate at least 50% of their needs/wants they get to move to an older room.
I currently have a 2.5 yr old in the "oldest" room and I have an almost 3 yr old that still spend a majority of their day in the toddler area (cruisers/climbers) so it's rarely by age but more by development.
IME, it's the difference in developmental stages that causes the most discord. Grouping kids that way helps eliminate some of those issues.