if you are working then you should contribute no matter what age. I was working at 15years old and contributed to household bills and food. but ideally if your not working and dont earn money you should still contribite by chores :)
I would say, when the young adult has a good enough paying job. If he's paying for his own car and insurance, that needs to be taken into consideration. I think no more than 1/3 of his net income after taxes.
As soon as they have any kind of job, even a paper route. Contributing a few bucks towards household expenses helps to teach good financial habits. My first job at 14 was divided into thirds, 1/3 to help out, 1/3 to savings and 1/3 was mine to do with as I pleased.
Check out what renting a room is including utilities in ur area, Since ur Mama include food with it, and use of Laundry. If they do trade for helping out around house cheaper rent but has to be done before rent is do :) I would have them give me 10% off the top so you can save for them, and give it to them when they move out.