There is a twin "epidemic," at least in this country, although it's mostly fraternal twins (which is handy if you want boy/girl twins for your story anyway) instead of identicals, but there are more of both kinds than formerly and nobody knows why. So, in the near future, if this keeps up, teenage and adult twins won't be as much as a rarity as they are now, and will be too commonplace to seem like a cliché in books.
Until the last few years, I can only think of six sets of twin that I've been acquainted with. Since Jack and Ethan were born (and they were six in September) four other pairs of twins have been born to people I know or have met. Of these recent twins, only one pair is identical. So six pairs in 57 years, and then four in the last six. Also, Megan tells me that in Ethan's grade at his school there are three pairs of twins (that's four out of about 90 kids) all of them identical twins except for him. Jack goes to a different school because he's in a special ed class and Megan doesn't know about the other kids in his grade, but in his class (which is 12 children) there is one other twin.
Plus, it seems like several times a year some celebrity mother is having twins.
The increased number of twins is partly due to in vitro fertilization where more fetuses are planted, and partly due to pregnancies in older mothers, which increases the chance of multiple births ... and neither of these trends is likely to end soon, and may increase ... but more young women are having them, too, and as I said, no one knows why.
