All children are different and some need more emotional support from their parents than others. But I would venture to say that perhaps the families you know where the children continued to have trouble falling/staying asleep on their own, were families that were not consistently emotionally connected in all parts of their interactions. Sleep issues often crop up in people who are struggling with stress. My oldest daughter coslept with my husband and I until 3 1/2, and my 2-year-old is still with us. The younger one definitely needs more physical emotional connection, but I maintain a strong connection day and night and this builds up the trust that I will be there — if not in person, then in emotional connection — and helps the child form independence.

I did want to add that an excellent source of cosleeping education and support is Attachment Parenting International at http://www.attachmentparenting.org or through its Infant Sleep Safety Campaign.