Amira Hergal is a twenty year old woman who has travelled across the mountains near the Caspian sea in order to wed Karluk Ayhan, a young boy eight years her junior. Despite being far from her own village, Amira is a hard working girl who is eager to please and happily adapting to life in her new home. Now she lives a peaceful life with her new family, from going to meet Karluk’s nomadic uncle and baking bread to amazing the villagers with her archery skills. But when her old family decides to reclaim Amira and have her marry another man who will be more beneficial to the village, it seems her happy life with Karluk could be at an end...
When speaking of older female x younger male romance with considerable age gap there are two series that stand out. First is Otoyomegatari which has probably the best take on the subject in seinen manga ever. Second is Kumiko & Shingo which may appear similar to it's shoujo peers, but stands out due to it's length and the fact that the story spans many years.
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?
Both are shoujo romance series with similar age gap, older female x younger male. If you read one and liked it, the other should be right up your alley (keep in mind that MeruPuri has those fantasy elements, while Kumiko & Shingo does not).
Both of those series are fairly long shoujo about a high school girl x elementry school boy romance. Both have very oldschool art style and fairly good story. Kumiko & Shingo has none of the sci-fi/fantasy elements that Boku no Chikyuu wo Mamotte features, but hopefully it won't prevent you from reading one if you read the other.