Tsuji Motochika is a guy who loves little girls. Yuna looks like a little girl, but she's actually older than Tsuji. Also, she likes much older men.
Together, they make an agreement to show each other's greatest desires - Tsuji's father and Yuna's little sister - and in the process, learn that age is only a number.
...Okay, you read the title, right? Then you know exactly what this series is going to be about - it couldn't be more literal than that. No need to go so quickly, we won't judge you (though secretly, we will).