Yes, vitamn C is good for you. As an essential vitamin, one that the human body cannot manufacture or store, it must be included in your daily diet, acquired from natural sources, like citrus fruits and green leafy vegetables, or taken as a supplement. Adequate vitamn C is necessary for the formation of connective tissue, for the absorption of iron, and important in wound healing. Too little vitamin C leads to bleeding gums and general muscle weakness. Too much vitamin C results in diarrhea, nausea, vomiting, even kidney stones. The absorption of vitamin C by the gut is limited despite high oral doses. Excess vitamin C is excreted in the urine. Research has yet to show that ingesting mega doses of vitamin C can pump up levels of the vitamin in the skin.
Vitamin C is an important molecule as it plays a vital role in collagen synthesis, the main structural protein found in skin and other connective tissue. It is commonly found in skincare products with the hopes of repairing sun damaged skin and increasing collagen formation. Although few research studies do indicate that vitamin C containing skincare products (especially those with a very low pH) may reduce the number of sunburn cells on the skin surface after exposure to ultraviolet light, the majority of "oxidative damage" and wrinkle formation occurs in the lower layers of skin, in the dermis. Unfortunately, the ability of vitamin C to penetrate into the lower layers of skin is limited by its water solubility and extremely unstable nature. Vitamin C is oxidized readily in the presence of oxygen, and once it is, it is no longer an active vitamin.
Many vitamin C containing 'anti-aging' creams, serums, and gels are available. Many of them do not contain the most active form of vitamin C, ascorbic acid, and those that do, are often not formulated at the appropriate low pH that maximizes the effects of vitamin C (because at such a low pH products often cause irritation and inflammation.) And none of them, once exposed to air and applied onto the skin, are able to prevent the vitamin C from oxidizing and turning into an inactive yellow compound called Dehydro Ascorbic Acid (DHAA).
Consumers read the front of a skincare product and are content when it contains vitamin C. They do not know the pH of the product, as manufacturers are not required to put that information on the label. It is not evident when the vitamin C is inactive because of the wrong pH or because the form of vitamin C in the product has minimal, if any, benefit. In addition, until the product turns brown or orange, which may be difficult to see as many vitamin C containing skincare products are packaged in dark bottles (in an attempt to minimize exposure to light and oxygen), they can't tell whether the vitamn C has already oxidized. The consumer is happy with the product because 'contains vitamin C' is written on the front label.
As cosmetics, vitamin C containing 'anti-aging' products cannot legally claim to change the structure of skin or they would be classified as drugs and need to acquire FDA premarket approval (see Cosmetic vs Drug.) In actuality, vitamin C containing skincare products are formulated as moisturizers. The anti-oxidant and stabilizing effects the vitamin C has on the formulation itself may be beneficial, but it is likely the mositurizing ability of these products that appeals to the consumer. 'Anti-aging' benefits of topical vitamin C products remains controversial. And none of them have been shown to be as effective as sunscreen.