Consenting adults are consenting adults. The Bible is the word of men.
As far as what "consenting adults" are, I am not the judge of others' values. Go with what is legal in your area.
It's a tough call to say to teens that sex is illegal and immoral based on a line drawn by the government, but you can at least say how it is illegal.
I think an age difference factor is included in most of these laws. If sex with a 17-year-old is statutory rape, most jurisdictions view it different if the "adult" is somewhere within 2 years of the minor's age.