Anecdotal evidence is the next best thing to no evidence at all, scientifically speaking, but . . .
We have three kids: a nine-year-old son, a seven-year-old daughter, and a two-and-a-half-year-old daughter. All have shared a bed with my wife and (much of the time) myself from birth to about age three. Neither of the two older kids have had any problem sleeping in their own beds since we moved them over. Once my daughter was born, my son moved to his room. For the first few months, I’d put him to bed and stay in the room (sitting or lying on his bed, a futon) until he got to sleep. After I while, I started staying with him for a certain amount of time (five minutes or so), then leaving him on his own whether he’d fallen asleep by then or not. After a year or so of that, we started following a simple bedtime ritual that ends with me turning out the light in his room and saying good night. He’s always had trouble settling himself down to go to sleep, but he’s a sound sleeper once he’s out and has only asked to join us in our bed maybe 3 times in the last 6 years or so.
My middle daughter and my wife have also arrived at a nighttime ritual. She’s always been a lighter sleeper than my son, but she was also able to go to sleep on her own much more readily than he did. As with my son, she moved into her own room before her third birthday. She’s been known to join my wife in bed a little more often than my son, but no more than a few times a year.
Youngest one still sleeps with us, but we’re expecting to change that soon.
What works for us wouldn’t necessarily work for everyone. For one thing, I travel for work a lot of the time (40+ weeks last year, though it’s been less this year), usually for the whole work week, so I’m not there a lot of the time. Also, even before my travel load got so heavy, I often slept in the guest bedroom – the mattress works better for my back, and I had an undiagnosed sleep apnea problem that caused me to snore loudly for years. We were all more comfortable in many cases with me in a different room. In our new house, I have a home office with a futon in the basement, while our bedroom is on the second floor. So we’ve always had an alternative venue available for conjugal relations; I doubt our arrangements would have worked as well for us without that.
For another, my wife is a LaLeche-League-card-carrying-member/Mothering-magazine-subscriber type. With breastfeeding, it’s just easier if mom and baby are already in close proximity for those nighttime feedings. We’ve gotten used to just laughing it off when people try to convince us that our kids will still be sleeping with us when they go off to college. Neither of the older two are perfect by any stretch, but neither do any of the issues they have seem attributable to cosleeping when they were younger.