Hm I actually decided mine would be for young adults and up, after careful consideration of the themes. However, since my story is a fancomic, it will pull in younger people that recognize the show which creates some conflict since they're probably going to run into really dark themes that they might not be looking for. Expecting it to be cute all the way, so, the only thing I can do is warn them about it xD
But you know, it's really hard to control who reads your stuff, since for example... My art style is on the 'cute' side so it will attract younger people since the art style is already selling it to that audience, so, the best thing I can do is introduce the 'dark' elements little by little to deter those expecting something different (like only comedy and romance) and get the people that feel interested on the more mature subjects stay.
The fandom around my fancomic's show love shipping characters and romance, so I had to make a great effort to make the point early on that the story wasn't about that at all.
As for the reason I made this comic (it might help you think things through that's why I mention it), it's because in this show (The powerpuff girls) I was unsatisfied by how little the rowdyruff boys appeared, I also wanted to base the story around one of the episodes in which Mojo and Him are fighting over who is the most evil father and I wanted to write something psychological, as I was interested in fleshing out their personalities and inner worlds.
All of this out of the dissatisfaction from not seeing enough of them. However, I knew my story was going to be mature and people seeking romance weren't going to care for it, thus, I remember the first thing I did was to scare them from the beginning as some sort of filter xD
But anyway, you see, you have to first know what you really want to talk about in your story, the ideas in your story are what dictates your audience after all. Just like in the case I described above, if you're talking about mature subjects that won't be appreciated by younger audiences or you know younger people will have a hard time, then you have to consider targeting an older audience.
So in my case, I would suggest you to consider if the themes in your story are relevant to children, teens, adults.
One might want to market it to everyone possible but in the long run, you'll feel limited, since you can't show the same things to children than teens for example. Think well what you feel passionate talking about in your story and think well which audience would appreciate it the best.
I think that's the best I could say about this, hope I helped somehow!