I did learn, the older you are the more likely you are to have twins.
I've never heard that stat before. I'm not doubting it, but I think I can probably explain it in part:
I'm guessing that more older women use IVF, which in turn increases the chances of twins.
I'm not sure that the biology changes (though it may very well do so) but I'm pretty sure that all those IVF's play with the statistics.
It is true that older women are more likely to get fertility treatment (not just IVF) which increases the chance of multiples.
But it is also true that twins are more likely with greater maternal age alone.
From http://www.gynob.com/obpre.htm
Identical (monozygotic, or twinning from a single egg--"identical") twins happens once in 250 pregnancies. This is unaffected by age, race, or any other factors. Dizygotic twins (twins from fertilization of two eggs), however, is a different story.
In dizygotic, or "fraternal," twins, not only maternal age, but the number of previous pregnancies ("parity"), increases the chances of twins. Twins are three times as high in women over 35 with at least four other children than women under twenty pregnant for the first time. Taking parity alone, the chance of twins doubles from the first pregnancy to the fourth pregnancy. Racially, African-American women have a 1 out of 79 chance of twins; caucasian women 1 out of a hundred; Asian women more rarely--1 in 155 pregnancies.