Help her develop habits that will last a lifetime.
Aging beautifully starts when we are very young with good eating habits and a good hygiene routine. So developing good skin and hair care habits should begin early. When playing with Mommy's make-up becomes "when can I wear make-up", it's time to commence encouraging grooming habits that will last a lifetime.
Whatever age you set as the proper time for using make-up, give it a good six month head start by letting her demonstrate her developing maturity by twice daily face washing, using body lotions with SPF and lip balms. Showing you she will take responsibility for herself in these small items will help you to measure her willingness to maintain her skin once she is allowed to use more grown-up cosmetics. After all, if fried fast foods are her snack of choice and make-up is allowed to remain on overnight, it is very likely she will develop acne or other skin problems. She may not thank you for the strict regimen right away, but she will as a teen when her skin remains clear and when she is turning sixty and still looks fifty!
Make it a fun experience and a challenge with a reward. Shop together for her first "grown-up" products. Discuss the importance of skin and hair care. It is hard for someone so young to imagine herself "old" (and remember even you are old to her), so you might want to find a few pictures that demonstrate women with older, uncared for skin and a few with skin that has obviously been well-cared for. Challenge her and offer the reward:
"You are growing up so beautifully. I know you want to start wearing make-up and you will be allowed to [..when...]. It is my job to teach you as you grow and your job to show me how responsible you can be. We will start you with [...]. If you can show me you will wash you face every morning and every night without being reminded, and you will [...] for the next [...] months, then I think you will be ready to begin using [...] ."
Be sure to get products that work well for young skin. Go for the natural products in a good quality face wash such as DermaE's Tea Tree and E Face Wash with it's natural antibacterial ingredients. Remind her it is better to rub gently for a little longer than to scrub hard quickly and to blot her face dry.
Moisturizers should be used daily. This can be part of the shopping fun. There are lots of high-quality natural moisturizers that are naturally scented. Let her choose her first "signature scent". Smart Spa by Qtica it great! Choose a good lip balm; one with excellent moisturizing and SPF qualities like BodyDrench...it does have the "candylicious" flavors, but these are infused with Beeswax, coconut oil and vitamin E.
Finally, and most important, encourage her good eating habits. Start packing her lunch with a few carrot sticks instead of two cookies. Let her go to the grocery store to help you pick the healthy snacks. Stress the importance of eight glasses of water a day for a healthy looking skin. She should come away from this experience feeling good about herself and you will know she is on the road to a lifetime of healthy skin care.