Naisho no Tsubomi

Both is about kids going through puberty and falling in love.  

Both series contain comedy, drama, and romance that takes place in a school life setting. Both series' animation are also similar and the main male protagonists are popular with the girls. 

Some of the flashback scenes in Only Yesterday happen almost exactly like scenes in Naisho no Tsubomi. Both involve young girls learning more about crushes, relationships and even their own bodies and of course endless teasing from their male classmates about these issues. 

both contain element of awkward first encounter with romance and sexuality araburu kisetsu is told from the teenager viewpoint  

-Both give off similar feelings, -Both involve romance, school life, cuteness -Both ova's 

Both are about growing up and dealing with the change. 

two anime about starting to grow up (romance, school, etc.) slow-paced focused on characters and relationships rather than action 

I immediately thought of Naisho no Tsubomi when i watched 12-sai. Both anime are about the main character going through and experiencing the change from a child to a grow up. Naisho no Tsubomi mc is 11 and 12-sai mc is 12 years old. 12-sai is more focused on falling in love, while each episode of Naisho no Tsubomi is focused on general "growing up" (falling in love, puberty, becoming an older sibling) 

Well...Shugo Chara is sweet and funny. If you like Naisho no Tsubomi I bet you'll like Shugo Chara!!  

Both are about the importance of bonds. Both deal with the main protagonist meeting someone someone from her family that cames from the future. Both are cute animes to a younger audience.