If they are with their female mother, then they can go swimming, even within 5 hours of hatcing. The only important thing is the oil that helps the ducks swim.
To how old can they be outside and roam as they please? Ducklings are much stronger than chicks or guineafowl keets. As long as the mother won't free ranger far from home, you can let them out between 7 to 10 days. Let them go outside, let them go taste the delicious grass, let them go chase the insects at edge of the pond, but that you'll be teaching to know the real world. And sound the alarm if danger when a falcon passes by.
Ducklings are easier than chicks, since they don't need too much care. But when letting them out. The weather must be pretty warm.