You can actually begin potty training a child at age 18 months which is a good age to start introducing the child to the toilet.
They may not grasp the idea or start using the potty that early but it's great to start at age 18 months because surprisingly some kids are ready to begin toilet training at 18 months like I was.
My mom tried potty training me at 18 months I took to the potty so you can give it a try.
If the child doesn't seem interested at 18 months stop and wait until the child is 2 years of age to try again.
If the child hasn't potty trained by 4 years of age you need to really start forcing the issue.
As long as they're 4 and under it's not a big deal for the child to still be in diapers full time and not potty trained but once they do reach 4 years of age they do need to be at least going to the potty.
If you have too just remove the diapers and put them away where the child can't find them and tell them no more diapers and stick to it even though they will have accidents in their underwear or on the floor.
Keep pushing the issue of the potty and they will eventually go to the toilet even if they throw a tantrum over it.
It's just part of being a parent dealing with that stuff.