What are the requirements to be a face model??

I'd want to be a nude model but i think i'm a little too young for that (16) and i'm only 5'6 so i can't be a runway model. I was thinking about being a face model so can someone tell me the requirements for that. like what they want to see in a face, symmetry, nose structure, teeth, eyes, facial shape, hard or soft face. thorough answers please. your help is greatly appreciated. :)))