Nokorimono ni wa Fuku ga Aru.

Nako's been summoned as a sacred figure, but she doesn't have the special powers her summoners expected to make this "other world" a better place. She's sent away to live in a detached palace with Lynn, her lady-in-waiting, until one day, she's ordered to marry an earl who's said to be the country's hero. The thing is, he's over the age of 60. However, Nako takes one look at him and instantly falls in love with his gentle, reserved nature. Although this hero is aware that he's old and doesn't have much of a future ahead, he can't stop being attracted to the bright, cheerful girl. Little does he know that he will soon wake up and find that he is back in his twenties!

(Source: Renta!)