You can offer the calves hay at about 21 days of age. Digestion of solid feed can begin at that time. Second cutting hay will probably be better. You can offer starter feed at the same time, but begin with small amounts (2 to 3 cups). You should increase the starter feed very gradually so you don't cause a digesative upset.
Milk replacer should be feed for 3 to 4 months. You can stop feeding milk replacer when the calves are fully adapted to hay and grain.