Not QVC models. they reflect their audience. From about 30 to 60. Generally high fashion models start at 15. Some at 13. If the 13 year old is tall and thin, & her breasts have not fully developed....then she's fine for high fashion. One reason girls start very young in modeling. She can model under 18, just that usually her mother comes with her. Kate Moss started at 14, Gemma Ward at 13, Naomi Campbell started at 16 (she's second generation model, as her mother was one). Usually the most successful model started under the age of 17.
Per Kate Moss. She's only 5'6" but she reached that height when she was 14. She was rail thin too. The modeling agnecy thought she would grow in height, but she didn't. That was the reason she was signed to an agency at 14.