Languages need a lot of upkeep if you want to keep speaking them fluently. On the other hand, just like muscle, once you've had it it's a lot easier to get back than having to put it on for the first time.
My husband's mother, a German who spent a few semesters in Britain in the late 50s and subsequently taught English and geography in gymnasium (German academic track middle and high school), taught him and few other neighborhood children some English during her maternity year after his younger sister was born. He was 4. She then went back to work, he went into regular German Kindergarten (preschool), and the whole matter was forgotten.
Until he was 10 and started classroom English in 5th grade - he had a very easy time of it. That year of getting English sounds into his little kid brain, despite coming from a non-native speaker who had only spent a few semesters in England, did some sort of magic, because ever since I've known him, he's sounded British enough to fool Americans (British people, on the other hand, can hear that something's off, and of course can't place his accent). He's a more fluent English speaker than I am a German speaker, but we both have to speak more English at our jobs than German.
