Most times for most positions at Mcdonald's you need to be 14 years old to work at Mcdonald's.
But some Mcdonald's locations may hire you at 13 years old such as for being a Cashier if you can handle the money properly.
I've seen a few 13 year old's at different Mcdonald's locations working as cashiers and some taking out trash etc so you might be able to find a job as a 13 year old at Mcdonald's locations.
But if they won't hire the 13 year old they will usually always hire a 14 year old as long as you have a good work ethic and are staying in school etc.
Mcdonald's is a pretty good and easy place to get hired at while in school and you can work your way up to a better job at a different place.
But Mcdonald's is a good place to earn money especially while putting yourself through college.