As above, you wait until showing signs and has the vocabulary to communicate when soiled, wet etc Usually around age 2 to 2.5 years
Potty trained my eldest just after he turned 2. There are signs of readiness, which can make the process easier, but there’s also lots you can do before to get them ready. You also don’t have to wait for them to be able to talk/tell you- plenty of non verbal or SEN children will toilet train without the means to verbally communicate a toileting need, or plenty of younger children yet to develop expressive speech. https://eric.org.uk/potty-training/
You start when they are starting to show awareness of needing wee or poo. Making them start too early can confuse them. To start getting them to associate nappy change and toileting you can start doing all nappy changes in the bathroom and getting them to wash their hands afterwards to help with a routine