Younger people are statistically more comfortable with mixed marriages of all kinds, including race and religion. So it seems counterintuitive that the likelihood of marrying someone of a different worldview actually increases with age at marriage:
• Married between age 16-25: 48% are in mixed-belief marriages
• Married between age 26-35: 58% are in mixed-belief marriages
• Married between age 36-45: 66% are in mixed-belief marriages
It makes more sense when you consider two things: (1) The very religious often marry young, and (2) the older you are, the more likely you are to have mingled with people of different religious beliefs. Your pool of potentials tends to get much more diverse during and after college (or once you start your career) than it was when you were 18.
(Stats from Riley Interfaith Marriage Survey, conducted for ‘Til Faith Do Us Part by Naomi Schaefer Riley, Oxford Univ. Press, 2013.)