> I'l possibly try writing something more lighthearted in the future.

I'll stay tuned then :-)

> As for the age, most of the girls are in their high teens

which makes them practically adults. They don't attend school and instead have proper jobs, live on their own and have all the adult problems and responsibilities, and they face them behaving as adults. The art style also doesn't try to make them look younger like it often does in anime which makes the viewer used to anime automatically assume the characters are older then they look. Of the main cast only Caro and Erio feel like children, but even they feel older then their official age. I was sure they were about 12-14, but looking it up it appears they were supposed to be 10 in StrikerS.
But I guess it can't be helped that each of us has their taste for characters' age.

Anyway, the story is good and I'm looking forward for more.