Well the way I see it is that some people wear diapers for medical reasons, others for fetish reasons and others simply for comfort or confidence reasons. Whilst they can be, these reasons aren't necessarily mutually exclusive and can frequently overlap. For what it's worth, which probably isn't a lot, I don't think there's anything inherently childish about doing something which provides comfort, pleasure, and probably makes your life easier, so long as it doesn't harm others.