The 1yo stage is surprisingly convenient for that sort of stuff. At ages 2-4 they're much more mobile and experimental and harder to watch. Then from 4 or 5 upwards they're at school a lot and starting to lose interest in what their parents want to do.

Also, the first 18 months establish a good deal of a person's attachment security, connection to parents, and emotional intelligence. The brain continues to be flexible for some time, but gradually becomes harder to rewire.