Sometime after 18mon
After 18 months of age, a toddler knows how to cooperate when held steady on a toilet seat.
Many parents in the U.S. Wait another year, because disposable diapers are so easy to use. The cushy toilet seats are a recent invention, and are not needed if a parent holds the child steady. A small plastic potty for kids if fine to use, and may work better because kids can rest their feet on the ground.