As early as possible! Language learning is extraordinary easy for small children, as most of their synaptic connections have not been pruned away yet, and their language centers are still fully active in their brains. Language learning skills decline and plateau at puberty for most people.
Babyhood. A lot of linguistic research indicates that children are able to distinguish their languages fairly easily, without getting them mixed up. And, as a baby, an individual can hear all sorts of phonemes, but this ability goes away soon after.
Obviously, Day 1! Absolutely nothing works better than total immersion. My son was born in Spain, and we had a lovely Spanish woman who helped out as housekeeper and occasional nanny. My son's first spoken words were Spanish. No, he had no difficulty with English, either.
I think this is a really great idea if you have a native speaker on hand to immerse them. I think kids are very good at associating language with a specific person - that's how they become bilingual. My problem is that I'm just a languages teacher...