Studio Ghibli films are definitely not only for young audiences, as this film deals enough with real-life issues. I watched the movie 2 days ago and it still strikes me today. From Up on Poppy Hill is one of Ghibli's most underrated films.
It's sweet and painfully beautifull. Yes, it doesn't deal with the wizarding world like most of the Ghibli films. But this one deals with human aspects of life, such as grieving over the loss of a loved one, the effect of wars and first love.
Shun and Umi faced a delicate situation where they might be siblings who felt in love with each other, but I appreciate how they both handled it in a mature way, despite the pain it caused them.
The movie is pretty serene, beautifully relatable, and a pretty sad story too, but it's one of my favorite Ghibli movies.