DS coslept from the start. A few futile attempts at getting him to sleep in a baby hammock, or the sidecar cot... He made it clear he'd prefer cosleeping, so we did.
He was a terrible sleeper. Terrible. He'd wake every 45 minutes, and scream. Cosleeping was the only way for me to get any sleep at all. Lots of people blamed cosleeping for the problems. When he was 3(!!) finally had his toncils and adenoids out, his sleep improved overnight. Really, like a miracle, it was.
It turned out he'd had sleep apnea, so that screaming was him panicking about not getting air. Of course, we had three years worth of habits to break, but a year later - he sleeps in his own bed, in his own room and pretty much always sleeps through.
He was ready when he was able and ready.
There were so many times when people tutted about rods and backs, sometimes I doubted... But turns out we hadn't ruined him 
DD just turned 2. She has her own room, with her own bed, ready an waiting. She's very excited and sometimes tries it out - but says she wants to sleep with me anyway.
Any day now, she'll move to her room, and when she does, I'll know she's ready. I don't doubt "the system". 