Ds now 3.5 still semi cosleeps. It seems to be when there are 'problems'!
At 20 mo he wanted to sleep in his own room and if my boobs were detachable he'd have taken them in. However they're not. So he came in at first wake up as I got more sleep that way.
Gradually he lasted longer and has recently been sleeping around 10 hours, which is all night if he's had a nap in the day.
He had a ridiculous number of illnesses caught from nursery in the first 3 years (touch wood lots less now) inc scarlet fever TWICE, so tbh it was easier to be consistent rather than try to battle staying in own room.
Partly due to these and partly as DH works long hours he is very clingy to me, esp at night (that's slowly changing).
I actually would have found it easier if he started off in our my bed tbh.
Incidentally, one of our non sleeping so want to cosleep 'problems' at the mo is when he somehow doesn't need a wee at bedtime then does in the night but is too good at holding on to it (will not go) and is uncomfortable, then we have an accident around 5 am. It's only happened 3 times but recently when it's been v hot and he's been v busy during the day. Others have been nightmares, sore throats, overly busy day.
Sarah Ockwell Smiths gentle sleep book has great ideas for helping toddlers and preschoolers, one of which is spend time playing in their room everyday. And red / orange lights in Eve and at night. I think both have helped ds a lot.