It may have been in the books, but I believe Rhaegar is/was ~15 years older than Dany...perhaps it was even more than that. I'm not certain now.I'm still confused on the timeline regarding Jon's parents. I didn't think Rhaegar was old enough to be near the same age as Ned Stark. I thought it was the Mad King that wedded Lyanna. Then JS and DT would be cousins, not aunt/nephew.
Then you fast forward to Dany and Rhaegar's exile and they appear to be no more than 10 years apart.
EDIT: I think I was way off. Here's a good reddit discussion on the matter:
The first response says: "Rhaegar was born about 39 years prior to the start of the story, and died during Robert's Rebellion at age 24. At that time, Viserys was 7 and Daenerys had not yet been born.
By the start of the story, Viserys was 22 and Daenerys was 14 in the books. They aged one or both of them up in the show."