Well I believe the personality is a biologically occurring phenomenon, and that one is born with all of their cognitive function hierarchical "wiring" already in place (So for instance, while an INTP's Fe might be underdeveloped, it is always existent.). I don't think all cognitive functions develop at once of course, they are very much muscles in that an infant is born with all of them, but they must be used before they can develop. With all that said, I would say you should technically be able to discern their personality type as soon as they are old enough to articulate, maybe even earlier than that. But I should note that you would have to have a very high level of mastery to be able to recognize cognitive function use at such a basic level. So it would be optimal to type a child after they have had a chance to maybe go to school and be around other people for about a year at least.