The youngest I had to start treatment was 3 years of age after all intense sensory therapy, psychosocial management and parental courses failed rather badly.
This was to prevent the little boy killing gold fishes fished out of from the bowl, pushing down his little infant sister down the stairs, running away and disappearing in the malls/ London tube station, waking up at 5 am opening the front door and digging holes in the garden to pee in it, being EXCLUDED from nursery after nursery (mum gave up her work).
Basically to prevent endangering himself or others. Medications did help!!!