I personally learned to read as early as I could recall (around four) but lacked much attention span to actually crack open a book then and found being read to more fun. Not as a matter of personal bragging but to point out that kids in absense of formal teaching methods may absorb many things.

Immersion learning is used and studied mostly for foreign languages but I wonder about how effective it proves in general for small children given vs trying to teach them actively and leaving them to observe it. It may be a fringe case technique when directly teaching them is ironically less effective or even more effective when it does work but unreliable.