How many months can baby be trained to sit with support?

No training is done: A normal baby's nervous system (brain, spinal cord, nerves) and his muscles will mature in a head-to-tail pattern. The neck will develop its strength, control, and coordination first. Then come the arms and shoulders. Next are the chest and abdomen. Last are the hips and legs. In order to sit alone a baby controls his abdominal muscles (age 6-8m). Later he'll crawl by controlling his leg muscles.