For me it's not a question of age but mobility/independence. I have a year old that walks well, climbs stairs, self feeds. But, I have an 18 month old (she leaves March 1) that does not walk and is a very heavy child to carry. That type of child (like my 18 month old) is more taxing to your body (your back from carrying, lifting, body position if having to spoon feed).

Here is my current breakdown:
1 @ 3 years old (my own son)
1 @ 3 years old (girl)
3 @ 2 years old (2 boys, 1 girl...the 2 boys share a full time spot while Moms on Mat leave, one Mon/Fri, the other Tues-Thurs.)
1 @ 18 months (she will be leaving March first; when a 1 year old is starting)
1 @ 14 months (walks well, climbs stairs independently, self feeds)