With children it's a developmental window that opens and closes in the first few years. The lucky little bastards learn languages organically.

For the rest of us, it's a long, hard slog to learn a 2nd language. I understand that it is easier to learn a third language (even if unrelated to the first two) if one has a command of the first two.

I expect it's related to the different ways that children and adults learn language, I dunno.