A toddler is a child that is between 12 months and 24 months. A toddler bed that uses the baby bed mattress is not made for kids that are larger than a 24 month old child. If the child is on one by 18 months and is sleeping comfortably they can stay on it a bit longer than their 2nd birthday. But if they are heavy chances are they will be uncomfortable and keep waking up all hours.
It is better to move an older child directly to the bed they will age out in instead of trying to move them too late. My niece moved each of her children to queen beds at 2 yrs of age. That is what their room size could comfortable hold. The kids went in and slept fine.
My grand kids were all in the toddler bed by 15 months and by 18 months they were sleeping full time on it. They went to a regular bed by 2 1/2. They were not large kids so they were not uncomfortable.
In child care baby's start taking their naps on cots at about 10 or 11 months so that when they transition up to the toddler room when they turn 1 they will be used to sleeping that way. The baby room also gets them down to one nap per day. With all my years in child care it seems to me that this 15 -18 months is the ideal time to have them fully used to sleeping on the toddler bed.
They younger they go to them the less likely they are to realize they have a lot more freedom. To the younger child it's just bed and sleep time. To an older child it's freedom to get up and go play.
I never had that issue but all my friends who waited until the kiddo's were 2 or older had to be up and down and up and down all night with a child who wanted to get up and play. I never ever had that issue with non of the 7 grand kids. They went to bed and slept.