Now. It is never to early to teach them what te word no means.
Learned behaviors. Typically start at 6 mos., the time to baby-proof your house. Remove a 6-9 mo. Old from harm, saying "no" as you pick him up from behind & re-direct him. Always pair "no" with a consequence; e.g., remove a spoon thrown by a 9-11-mo.-old while saying , "no throwing." 18-mo.-olds can understand time-out. Tell your child what behavior you want calmly but firmly, in a low voice, at his eye level.
Around 12-15 months. A lot depends on when your child starts to comprehend and interact versus just swatting at things. When they start to seek your approval and interaction, then a quick, firm no and a removal of your attention for a brief period can start behavior modification, and stop children from some behaviors like hitting and biting. A child who is too young to understand won't make the connection.