When do the terrible twos start?

Depends. The twos don't have to be terrible- they are a time of incredible learning and development. But they are also a time when children begin testing their limits and rules. This begins at different ages with different children. Look at this time as a way to foster growth in your child. Set rules surrounding safety, but allow them the ability for creative play and lots of free time.
Learned behaviors. start at 6 months. Toddlers need calm, consistent discipline: immediate praise for "good" behaviors & ignoring of "bad" behaviors. Neurotypical 18-mo.-olds respond to time-out & learn to self-calm. Negative or positive attention reinforces toddler tantrums. Model behaviors you want; he'll do what you do. State rules at his language development levels. Read "Little People" by Christopherson.
Around two! The "terrible twos" are a developmental phenomenon rather than a direct cause and effect by age. As children develop, they develop a sense of self that sometimes makes them seem resistance. Don't worry, its just a phase. Enjoy it along with everything else that is new and wonderful about your toddler.