I endorse aquinas' post above. Age-peers are often detrimental in social development, not helpful, and that's particularly true in the t-stages (toddler and teen).
Toddlers are only just venturing out into the social space, and have no idea about norms, boundaries, etc. So if you place toddlers together, it becomes a group social experiment, based largely on ignorance, and develops in ways that can't be predicted (though you can bet on hitting being involved at some point). The ideal is that children come into the experiment having already been given a solid grounding in acceptable social behaviors, and the best source for that is the parents. And the best way for parents to teach that is through engaging in child-led play.
As aquinas has indicated, young children tend to gravitate towards playing adults, even strangers. I don't understand it, but I've experienced it many times. DD has acquired a good number of playmates at the park who initially expressed no interest in her, but tons in me.
