Treat your date with the respect and purity you hope your future spouse will have. Keep in mind how you will expect a young man to treat your daughter one day. By listening to your conscience in this way, you'll have a good idea of where to draw the line.
If a child shows himself incorrigible, he should be decently and quietly beheaded at the age of twelve, lest he grow to maturity, marry, and perpetuate his kind.