Love can be difficult at any age. That's the theme of a new tvN drama "Nine Boy," also called "Age Ending in Nine Boy. The story features four characters whose ages end in nine.

All of the characters face a different challenge in their love lives. The nine-year-old, 19-year-old and 29-year-old are brothers. The 39-year-old character is still a bachelor

BtoB singer Yook Sung Jae plays a 19-year-old athlete. He falls for a girl played by A Pink rapper Park Cho Rong. She's pretty and mysterious. They cannot avoid each other so they decide that fate meant for them to be together.

Yook Sung Jae can be seen in the dramas "Monstar" and "Reply 1994. Park Cho Rong appeared in the comedy "All My Love" and had a cameo in "Reply 1997."

Kim Young Kwang and Kyung Soo Jin play the 29-year-old lovers. Kim Young Kwang, who appeared in "Good Doctor," "Birth Secret" and "Secret Love, is the oldest brother and a travel agent. At the age of 29 he begins to reevaluate his life. Kyung Soo Jin plays his co-worker and a good friend. Kyung Soo Jin played the young Son Ye Jin in "Shark," had a role in "That Winter The Wind Blows" and played Yoo Ah In's friend in "A Secret Love Affair." The actors have played a couple before, when they appeared in the music video for Brown-Eyed Soul's "You."

Oh Jung Se is a 39-year-old producer with a bad temper. Not only is he not good at people skills but he's also a workaholic, which leaves him little time to date. Then he meets his ex lover, played by Yoo Da In. Now she is a single mom whose life revolves around caring for her daughter. She will play the same character at ages 29 and 39

Oh Jung Se played Yoon Eun Hye's brother in "Future's Choice" and had roles in "Missing You," "Miss Korea" and "A New Leaf." He's also currently in a few films, including "Man In High Heels." Last year Yoo Da In appeared in the film "The Suspect."

Last but certainly not least are the nine-year-olds and they are likely to steal the show. Choi Ro Woon, who had roles in "I Need Romance 3," "Good Doctor," "Golden Rainbow," and appeared in the film "Miracle in Cell 7," plays the boy. His character is a child star that does a lot of food commercials but a rival threatens his career. He secretly dates the girl, played by Lee Chae Mi. She is the actress who charmed audiences in "Two Weeks," "Word From A Warm Heart" and "Golden Rainbow."

"Nine Boy" will air in August.