I think it is a cultural thing now, in that it is the norm for children if they are not toilet trained at a young age. Physically, most children are able to recognise when they need to go to the toilet by 18months, before that they are usually unable to control their bodies and are unable to recognise the cues. Of course there are children who are either side of the 18months age who can toilet themselves before or after but this is the general age when most children *should* be able to do it.
With younger children there are cues that we as parents could watch for and take them to the toilet, but TBH I just don't have the time to be constantly, closely watching my 5mth old to give me cues so I can take her to the toilet in time.