If I am not mistaken, the bag of food says around 20 weeks. However as soon as I add my young pullets to the laying house, they start on pellets which is around 16 weeks. But that is my preference. Not sure about others.
Sounds like you are feeding them a good balanced diet already so some scrapes will be good as treats. If you want good egg production, you will soon switch them to a laying pellet or crumbles and then I would limit extras. If you are not worried about their egg production, then the sky is the...