How to stop her from playing

My niece is 14 years old now. She is getting bigger and taller, taller than me to be exact. I am happy that is growing faster but I am sad because she still likes to play with kids. I have done my best to stop her from playing because it is not good to look at. Her playmates are smaller than her. She is in high school and her playmates are grade school. I discourage her to stop her from playing but it does not work. She even likes to play more. She likes to play with kids younger than her than kids at her age.

I give her new responsibilities at home to divert her from playing but it does not. I told her parents about this and they just told me to let her do what she likes, one day she will stop on her own will. When will that day come?? The question that only the time can answer, and worse, it is not sure if it is sooner or later. I really hope that the niece will realize sooner. When I was at her age, I already know how to cook, do the laundry, clean the house and do household chores. Things that I want the niece to do, but no cooperation from her parents. If only I am her mother.LOL Well, I will just give her time and let her enjoy.