I just checked my favorite twinning website, and here is what it says about the upward trend followed by the downward trend. It's more accurate than what I said above, though the general idea is the same:
"Mother's age. A 35-year-old mother is four times more likely than a woman under the age of 20 to have fraternal twins. After age 35, however, the chance of having twins naturally decreases."
So although you are still in the range to have a higher chance of fraternal twins than you would have at younger than age 20, that chance will have peaked already and begun to move to a downward trend.
Your chances at 38 of getting pregnant, without assistance of an R.E. and without doing IVF, are reduced, because of your age. (You can google this and get a lot of charts with depressing downward-trending lines on them.) But if you do get pregnant, your chance of twinning is greater at 38 than it was when you were younger. However, after you reach 45, that twinning-chance thing starts to go down again.
That really depends on a lot of factors. Race, genetics, fertility treatment (if any), age, even state.
I don't have a clue how to answer this. Anyone else? Good luck to you in trying to conceive.