I don't think there is a dividing line of any sort. In fact, I don't think there is such a category. I think that some Christians are more prepared and mature than others, but it isn't a clear line that separates them from the rest of us.
Being a Christian is a journey, a courtship with God. When does a person who's on a journey stop being a tourist? When they get home. When does a fiance start being a spouse? When they get married.
So we are all "new Christians," moving toward the maturity that life, hard work and God build into us.