A Study published in 2018 suggests that kids should stay in their cot until the age of three.
The results of the study published in the Sleep Medicine journal, suggest that if you want your child to sleep better, keep them in a cot to their third birthday.
The study examined sleeping patterns from kids from the UK, Australia, NZ, US and Canada,
The caregivers of 1983 toddlers aged between 18 and 35.9 months of age uploaded sleep data via a smartphone application. Of those toddlers included in the study aged between 18 months and 23.9 months, 63.4 percent still slept in cots. Cot sleeping was less likely as the toddlers grew older, with 34.30 percent aged 24-29.9 months sleeping in a cot. The figured decreased again with the oldest age bracket, at just 12.6 percent of toddlers aged 30-35.9 months sleeping in a cot.
The parents/caregivers reported better perceived sleep outcomes in those that were in a cot.
Toddlers who slept in cots had an earlier bedtime, fell asleep faster, had fewer night awakenings, remained asleep for longer, and reported fewer sleep problems and less bedtime resistance.
The only factor that did not change whether a toddler slept in a cot or a bed was the duration of their night awakenings.
The article reported that, "deferring the crib-to-bed transition until age three years may benefit toddlers' sleep in Western contexts".
You can find a link to the article here https://www.sciencedirect.com/science/article/abs/pii/S1389945718305707