We coslept in the "fourth trimester"
Cot in our room at 3 months ish
Cot in own room at 6 months ish.
We never left them to cry at all. I would say that they were 2.5 ish and in their own toddler bed by the time they tended to reliably sleep through.
There weee regressions (14/15 months was one) when for a few weeks we reverted to cosleepiing as the only way for us all to get sleep.
By the time they were 3 they were a nursery daily, eating three good meals daily plus snacks, and tired at bedtime. Then they started to sleep through without difficulty like an older child.
They're 14 and 12 now and have been brilliant sleepers every since.
And they are easy kids too - yes they have their moments but they're well regulated and can be reasoned with. If I say look I'm tired, please can you just cooperate, don't let's have a huge scene - put away your washing (or whatever) and we can all enjoy the rest of our evening! they usually understand and do it.
This is only my theory but I think taking relational approaches rather than training/behavioural approaches in infancy leads to older kids who can operate relationally.
We (esp DH) are quite firm - even strict - but we never used "time outs" nor "naughty steps" or other behavioural sanctions.
Nor do we confiscate phones or withhold pocket money and tbh have never had need to.
When they were little if they were playing up in a restaurant I would always frame it in a relational way - isn't fair on the other people here to make this noise, they've come to chat to their friends not to listen to two children arguing. If you can't stop we have to go home as we can't do that to others.
Might be proved wrong in later teen years but I think that going through the trenches in infancy and trying to keep relating to them as little people with big needs, helps produce kids that have that relational skill later on themselves and are easier to manage.
Mind you could equally be bollocks 🤣