2.5 years is perfect IMHO. Toddler is mostly toilet trained, old enough to help, reasonably independant, thinks the baby is the best thing in the entire universe, and is too young to be jealous.

Take a baby into a playgroup and all the 2.5-3yo toddlers will flock to the baby. I swear at that age they are hardwired to love babies. It is cute, but the little tykes are too young to be gentle and some try and pick the baby up, steal his dummy/blankets/hat, offer him 5,000 toys etc