That depends on the type of model. My mom was a hat model after she graduated from high school in the late 50s. She wanted to be a Rockette, but she was a few inches too short, but their was no height requirement to be a hat model. Just a pretty face.
There are many websites for the requirements for different types of models. Maybe this one will give you the basic ideas.
Most models are super fake! Try that :D
Really there's only one requirement ... To model something and get paid for it.