Uncommon, not rare. I would say that its uncommon, but not rare. Its hard to measure as there are many cataracts that are mild and not detectable or visually significant, unless examined by an ophthalmologist.
Usually. The majority of congenital cataracts are detected during young childhood either by poor vision or a white pupil. Certainly, cataracts may become visually significant in the adolescent years including some metabolic types. It's always a good idea to get at least a single eye checkup during childhood and more regularly if problems develop.