In both comedies a young school girl marries an older guy and their marriages are arranged by their grandfathers.
They both talk about an older man marrying a younger girl, even though my little bride was much fun to watch but both of them have the same concept.
Both have similar stories: young, high-school-aged wife with an older male arranged by their grandfathers.
The story line is similar - the grandfathers made a promise to let their grandchildren marry when they were older and the chemistry between the main leads is also somewhat similar (the bickering at least :p).
Although Sweet 18 is a drama and My Little Bride is a movie, they are quite similar. Both involves arranged marriage between a high school kid and an older guy which their grandparents chose for them. They are both very cute as well.
In both series the main characters need to marry because of an old betrothal made by their grandparents.
Both have arranged marriages set up by grandfathers. The bride in Sweet 18 is also in high school and have traditional families.
Both are about and older guy marrying an 18 year old for his family, and then eventually falling in love.