The usual question and answer seem to be of the form
What do you want to be when you grow up?
I want to be a singer when I grow up.
Should it not be
What do you want to be when you have grown up?
I want to be a singer when I have grown up.
Here we have interpreted grow up as the process at whose end one becomes a singer. Should the first pair be considered correct simply because of its accepted usage, or is there an alternative explanation for this?