I used to work at M.A.C
No you done need any training. You need to be good at make up thought.
They themselves seed you to classes and training seminars.
Your pay rate depends on location, as with all jobs experience.
Yes you can be eighteen and work at M.A.C
Holiday seasons are the best. They need extra help. If your good they keep you on full time.
Use brushes! When applying any time of powder even foundations use powder.
If you use dark metallic and light bases.
Practice on friends or yourself so you can achieve both dramatic costume style make up and flawless natural make up.
yes it pricey but use M.A.C. make up you will then better understand it texture.

well I dont work for M.A.C, but I know a few people that do.
1. Yes you do need make up training from a school.
2. Yes you can work there if you are 18
3. The best time I think would be around December since they are probably hiring more staff to help out with the holidays.

Hope that helped. Good luck