Unless a literary magazine/journal is looking for work from a certain age range (ex. the teen experience, 20 to 30-year-olds), I rarely see age restrictions under submission guidelines. If unspecified, is it safe to assume that most literary magazines/journals publish will work submitted by minors? I understand that competition is steeper for young writers, but I am asking this question from a legal/technical standpoint as opposed to a competitive one. Additionally, if unstated, do literary magazines generally accept international submissions, or are submissions often limited to their regional area (ex. only residents of the United States)? As a novice writer, any information on this would be extremely appreciated!
Neither age nor geographic origin are typical considerations when submitting work. Unless you specify them when querying or submitting, they won't even know (unless your email address and/or postal return address give it away), and you usually wouldn't specify unless it makes a difference to your writing, or supports your qualifications to write a specific piece.
When it comes to getting paid, however, it's a different story. I'm no lawyer, but I believe a parent would have to sign the contracts for a minor. And getting paid internationally is always difficult.
It's worth noting that the competition for inclusion in well-known literary magazines is incredibly fierce (even for adult writing professionals in the same corner of the world!). You might have better luck starting out by publishing in a local magazine, and/or one aimed at youth, and then building your way up to larger, more international publications. Better a successful publication in a small local magazine than any number of rejections from Granta.