We use this saying when parents or adults forgive children to do something because they’re too young.They say for example: You’re too young to do this kind of things; you will be able to do it when you will be older.
Given Spanish like to embellish this kind of expression, they use this saying. We can also use this expression for people, being new, who want to have privileges and whose we want to make understand that they won’t have anything until they won’t gain the confidence of the chiefs.
In olden times, the egg was a very appreciated and enhancive food, only father could and sick people (in order to get well) could eat a whole egg. For this, when the children asked if they could eat an egg, the mothers said: “When you will be a father, you will eat eggs”, the use of this sentence became standard for all the things we can’t do just because we are too young or because we don’t have enough experience.