I have a 110 page manuscript that I was hoping to get published into a book but I'm beginning to think 110 pages is too short. So I'm considering sending it into a magazine as a serial. Does anybody have any good advice?
If you can find a magazine who will take a serial from someone they've never heard of, why not? You might as well try anything that will take something of that length, few though they are. If your mc is young enough, it can also be published as a YA novel.
How old you are doesn't matter. A teenager with the right skills can write adult characters. And don't even consider telling the publication how old you are unless it is one that specializes in publishing work by younger people.