RE: the argument that the age of consent was ___, or women married earlier in the 18th century, here are some marriage dates for women who married founding fathers (presumably similar to Jefferson socially and economically--several were previously widowed):

Martha Washington: 27

Mary Paine: 19; Elizabeth Paine 22

Abigail Adams: 19 (almost 20; they met when she was 15, but he didn't marry her then)

Elizabeth Hamilton: 23

Dolley Madison: 26

Of course, we don't know the ages of women with whom these men had sex outside of marriage, but the pattern seems to be that 19 is a minimum age for marriage. Which is the point--Jefferson never considered Sally Hemings as a marriageable woman.