It does seem better to inform children either in their childhood years or in late adolescence, rather than during early to middle adolescence. Children seem to cope better when the disclosure doesn't coincide with their own struggles with identity and relationships. I lean toward disclosure during childhood. I know some fear the information may overwhelm or even scare children who are too young. But various sources indicate that it is never too early to disclose your LGBT identity.
Children take in what they can understand.
You should tailor your disclosure to their developmental level, both in word choice and detail shared. But there doesn't seem to be any minimum age. How Let me reiterate that there are multiple factors to consider in this whole matter. Besides custody issues, potential negative work and societal consequences, and personal comfort level, children's acceptance of parental LGBT identity may well be affected by the level of acceptance other significant people show. We can't control other people's reactions, nor can we keep other people from outing you before you can or do.