I had my 2 oldest boys share a room and they are 2 years apart when we were going through either of them coming out of their bed/room after it was their bed time we just kept getting up turning them around and marching them straight back to bed we had to keep it up for a few nights(even if they cried) but they learnt we were not going to give in so you MUST be consistent if you give in once they win and will know they can break you lol( so to speak) and we always used a specific word for bed time so they knew it was bed time we used nigh nigh time they boys both went to bed at the same time until they were in school and went with no fuss once 7:30 hit and i said nigh nigh time so teeth book and bed.

One exception was if they were sick they were allowed to sleep with mum and dad but that was the only time as DH and i value our alone time you need to draw the line somewhere and co sleeping is so uncomfortable i just cant handle it.
Oh and like minnimatron said 6am was when they were allowed in for cuddles during the nigh absolutely a no go zone.