As soon as they can sit still, my kids have been doodling and crafting. We maintain a constant supply of pencils and papers at easily accessible locations at home.
Painting requires more help, eg. Cleaning the brushes, changing water and mixing colours. As they grow up, they are able to paint on their own without much help. I just need to lay some newspaper on the floor and bring out the art supplies.
Mei likes to mix colours and paint on toilet rolls. I think it's the texture of the toilet roll which is slightly thicker than normal paper and the shape of the toilet rolls that makes it interesting for her. She will hold on to the roll, paint as she turns. She will use her favorite pink and explore new colours by mixing pink and beige, red and silver etc. It's sheer joy observing her paint toilet rolls. It must be therapeutic for her.