I'm not sure when, but at some age, I believe around two, though I may be incorrect here, (when I had some words of English) my Dad decided to speak Irish to me. In order to facilitate the switch, in addition to speaking it full time, would read stories in Irish every night. He told me recently about something's that he did. To teach me to say "bó" instead of "cow" he'd point at one and say "moooo-bó" or things to that effect.
My mother made the switch too, though her Irish was less proficient. In fact, she stopped after I was about 5ish, she couldn't keep it up
as a result of my Dad's efforts, my Irish was leagues better than my English until I was about six or seven maybe (when real life started polluting my brain