I see difference in beak/bill shape, so could be a barnyard mix. Looks like the black one is close to feathered out, the others not yet. So old enough to do without heat, although i'd be carefull if the nights get cold. They still have a lot of growing to do, so feed them as older ducklings.
Quick check says mature at 2 months, but i suspect that goes for having full feathers, not fully grown. So guess age wise is 5+ weeks.
