Perhaps the difference in attitude to planets and atoms is explained by chronology. We teach youngsters that the Earth is round because flatearthers have been a joke for ages, whereas it is still only decades since a scientist could doubt the existence of atoms without the risk of ridicule. Viewed in this way the philosophical objection to teaching atoms earlier is no more than an outworn tradition. The planet analogy also frowns upon the psychological objection, for no one feels that the Earth's roundness is too much for nine year olds to grasp. Instead of worrying about the 'cognitive immaturity' of young children teachers use an Earth globe and their pupils soon understand the reason for night and day.
Models are just as effective and just as essential for teaching atoms.
Planets are too big, atoms too small, but models of a cosy size make both planets and atoms accessible. The belief that an early start leads to more progress and a greater likelihood of continued commitment in later life makes music teachers for example demand young beginners. Children could begin their friendship with atoms and molecules at the same tender ages, when minds are at their most receptive. Puberty brings conflicting interests and an unfavourable climate for introducing the building blocks of science. It is not just the youth of the pupils but the way I bring in atoms that finds objectors.