The reason is you'll never have a child's brain once again. It's only in childhood that we can learn languages easily. As adults it is a hard work. But also lots of fun!
They told me that when a bilingual child learns two languges from his parents he knows them both equally good but cannot translate words from one language to another. Beacuse he doesn't learn one language on the base of another like we (adults) do. So the child surely misses all the fun of comparing languages, but we do not