Short answer: 13 years is nothing.
Longer answer: Got nothing to do with age unless you're concerned about what others think of you...it's to do with your heart and mind. You're a religious man Biteme, you know that according to the bible we as perfect humans were supposed to live forever, indeed the bible states that Adam lived till he was 930 years old so originally 20 years difference between partners would have been nothing. Now as imperfect humans our lifespan is written within our body code to three score and ten or there abouts but nothing has really changed apart from that our bodies break down and die much quicker. The mind does not understand this, scientists don't understand the reasons why, it just happens. Your earth age has nothing to do with how you act or think but societies view does. You'll know straight away if you're involved with a younger girl whether it'll work or not. Whether you're prepared to sacrifice intellectual conversation or common ground for a tight young body or mind blowing sex. Your heart will tell you and if your dick isn't thinking for you, your mind will guide.