You can (and should) offer a variety of prey from the first week of life. There is no reason to delay offering a variety of prey.
Mealworms should be gutloaded and calicum dusted and form no more than 10% of the animals diet.
Add also as many other prey choices as possible - silkworms, butterworms, terrestrial isopods, blue bottle flies, indian walking sticks, roaches, termites, etc