We wean our dairy calves at 4 weeks. They have high-quality starter, fine stem hay, and water available starting day 3 or so. Some even wean themselves, eat so much starter that they don't want milk anymore(doesn't happen much, but it has happened). Sometimes we will leave them on milk longer if it's cold and we have extra waste milk.

Been weaning at 28-35 days ever since I have been raising calves, 40+ years.