This is another one of those gut feel recommendations. Sure both series are seinen that feature a love triangle and both have mature content at times, but that would probably not suffice on it's own. There seems to be a familiar feel about those two (especially in the later volumes), but I can't quite name it. So if you liked one, you'll probably like the other one as well.
Airi has always dreamed of marrying her fated true love and living a normal, happy life – a dream disrupted forever when Aram, a prince from another world, emerges from her grandmother’s mirror. She takes the young prince home with her thinking he is a lost child, giving him a home cooked meal and a place to sleep. However, in the morning she discovers that, as the result of a magic spell, Aram transforms into a 17-year-old boy whenever it’s dark, and her kiss is the only way to change him back! If that wasn’t enough, Aram is in hiding from his older brother, the evil sorcerer Jeile, who has fallen in love with Airi and intends to make her his bride! Will Airi ever be able to find her happily-ever-after?
Age transformations via kissing; Thats the only thing Sakuranbo Syndrome and Merupuri have in common, but if you found that theme interesting, you know where to look.
P.S. Just a word of warrning: Sakuranbo Syndrome can get quite mature at times.
Both series seem to be targeted at similar (if not the same) audience, both feature an adult loli as one of characters and romance with strong mature undertones (read mature content, but Toshiue no Hito is much more guilty of that). If you read one and liked it, it's not very unlikely that you will enjoy the other one as well (just keep in mind Toshiue no Hito is even stronger on mature content than Sakuranbo Syndrome, but that's no surprise given the author).