What do you think is the ideal age to start a child in a preschool (or even daycare) type environment? By ideal I mean best in the child's interest, from the perspective of them being ready for it emotionally, having had enough of a 'foundation' at home first, to maximize the learning benefits from being in such an environment, and minimize any 'separation anxiety', or even just 'missing mommy/daddy'. (Obviously this is in a case where there isn't the *necessity* to put the child into daycare/preschool environments.)

Also, do you think it's better to start off part-time (2 or 3 days per week), rather than full-time?

My son started attending montessori when he turned 2 1/2 this Spring, for 3 mornings a week.  I noticed an immediate improvement in his verbal skills.  He went from speaking in a few halted sentences, to telling full stories.  A big verbal explosion. I'm glad we started him when we did.