It's a pretty common situation. It can come up at any age really. The biggest thing I can say is STOP IT NOW. My older two never did this (with one breif exception) because i was pretty strict on the whole sleep in bassinet, crib, toddler bed, and on.(they didn't know the difference)
I would nap with them sometimes, but they slept in their bed 100%. With my youngest daugher, I got lazy and often allowed her to sleep with us out of pure tiredness (she was a BAD napper during the day, so we were all usually pretty exhausted!) This was a nightmare that took till she was about 5 to stop. She is 7 now, and I still have her wandering into my bed in the middle of the night asking to sleep with me. I feel bad to turn her away because she tells me she had a bad dream, so I wind up sleeping with her for half the night. (very uncomfortable because she is a WILD sleeper) She doesn't do it all the time...only every so often now.
I would suggest not laying down with your child until they fall asleep. Sometimes it's worse because they wake up expecting you there for comfort. Your child has to get to the point that he soothes himself...not using your presence for soothing. I would have a routine every night that doesn't include you hanging around. But would include reassurance and tell him that mommy and daddy have their own bed, and he has his own bed. At this age, I would actually let him cry it out a bit. It really works. I used this method for a bit with my older child who for a short period of time, kept crying at night for attention. I would go in after 5 minutes, lay her down, give her a kiss and leave. Then if they cried again I waited 10 minutes and did the same thing. Then 15, 20 etc. As long as it took- before they realized that they were on there own and there was no reason to cry. You reassure them, but each time letting the time lapse become longer. It is painful to hear them cry and tiring for mom because this could go on for days...but it really works eventually. This was my only snafoo in my one older daughter's sleep habits. She soothed herself, and is 12 now....with no emotional scarring from letting her cry it out - I swear!! :)
I personally think that a child has healthier sleep habits when you nip it in the bud and get them back on track.
I personally slept with my mom till I was like in 4-5th grade. I look back on it, and realize how attached I was to the thought of having a body next to me. I think that was excessivly old, and it actually grosses me out at this point looking back.