1. Children can learn martial arts as early as 3 to 4 years old.
2. Children can learn to field strip and clean, tap, rack and fire a gun as early as 5 to 6 years old.
3. Infants can learn to swim even below 12 months old. (Well, they can learn to float and not drown.)
4. They can learn as many languages as you can teach them below 6.
Warning on number 3., I believe it's an "instinctual" thing that they will forget after like 1 year old, then you need to teach them
Agreed though it's amazing my 3 year old can manuever an ipod just as well as I can, to see his pictures and videos
There's a group of people and swim instructors for this purpose, that know about this, and actually teach kids between 1 to 3 years old, how to self-rescue and eventually swim.
I'm not talking about the infant that does it on instinct. You do need a qualified instructor for this though. I believe the group is called ISR. You can watch videos on youtube.