I have boy / girl twins who where conceived naturally 4 days before my 21st birthday.

I was told I have a really high chance of concieving twins again, about 1 in 8 and the chance will increase the older I get and the more pregnancies I have. Has anyone else heard this and does anyone know how they came up with 1 in 8? The only thing I can think of is that you must have 10 times the chance if you have twins and since the chance for the general population is 1 in 80, it brings it down to 1 in 8.