They are all okay grammatically, but the infinitive versions (1 and 2) show the process is starting. With the -ing forms, it sounds like they suddently just start talking, as opposed to it being an acquisition. However, language acquisition starts MUCH earlier than 3.
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MrPernicketyCould you tell me which phrase(s) are natural from your standpoint ?All four seem very natural to me. start and begin are pretty much synonymous, except you can only start machinery (cause it to start), not begin it.