"Our ancestors" did it isn't always a good excuse to do something. In the case of marriage, and by association intercourse, just because some of our ancestors married and cosumated at very young ages does not mean it is okay now. Our ancestors used to enslave people and deprive them of their natural rights, but no one would use that as an excuse to allow slavery.
In the West, 18 is the normal age of consent for the individual regardless of s**, class, or race. It is a period of time which people become full adults. It gives them legal rights and privileges to prepare them for full adulthood. Additionally, age gap is only an issue if one party is a minor. A 50 year old consumating with a 40 year old is not an ethical issue, but a 12 year old consumating with a 20 year old is.