The best age and weight to begin sleep training varies, since every baby develops differently.
Your child’s pediatrician can help you decide when the time is right. In general, 4 months and 14 pounds is a good time to start.
Babies do not actually develop their own melatonin or regulation of their sleep cycles until around 3+ months. Up to then, their ability to fully distinguish between day and night and connect sleep cycles to night is limited.
Generally, 14 pounds is a good weight because the baby may not need overnight feedings at this time.