You should start training homing pigeons when they are at least 6 weeks old. It's important to help them understand how to use the trapdoor in the loft at this age. The trapdoor allows the pigeons to enter the loft freely but can be set to only let them out when desired. It's also recommended to create a training schedule to follow daily for consistency and to ensure you're prepared for each session. This schedule should include training the pigeons at least once a day.