By age 2, we were laying with the kiddo in their room on a twin bed until they fell asleep, and then they were welcome to come in with us whenever they woke (unless it was like 10pm then we'd walk them back to their room and stay with them for a few min until they were back asleep) - and we specifically walked them through what they should do when they woke up. As they got older, that got later and later into the wee hours (we did have to tell them to just crawl in and not purposely wake us. lol!). When they were around 5, we started telling them they should stay in their room all night (unless scared or sick)...which is not to say they *did*, but we planted that seed. lol. They are 6 and 8, and we usually wind up with one of them in the bed a couple times a month at this point, which is still OK with us.
I think them knowing they were still welcome made the transition easier...it was a long transition, but was important to us to not make it a source of drama so we went at their pace with gentle nudges.
Then again, they didn't beat us up much while sleeping so it didn't really impact us much...if they were making it uncomfortable for us, we would have walked them back more and started requesting them to stay in their room sooner.