"Will ISKCON accept people this young?"
Oh, yes, we, younger, even younger. Even one child is within the womb of his mother, we can teach. It is so nice thing.
The instance is Prahlāda Mahārāja. Prahlāda Mahārāja, when he was in his womb of his mother, one saintly sage taught his mother about Kṛṣṇa consciousness, and the child became Kṛṣṇa conscious from the womb of his mother. So we can teach even in the womb of the mother because it is spiritual, it is not material. No material condition can check this teaching. Ahaituky apratihatā (SB 1.2.6).