As an unintended by-product of making films in ultraviolet, I've learned a little bit about skin. After How The Sun Sees You went viral, a lot of skincare companies wanted to know if I could do something similar for them. The standard answer is, that depends on what's in your product. A lot of products, like dark spot correctors, can't be shown to do anything at all. Some are borderline cases. Most had some scientific underpinning, but in order to figure out if it'd be demonstrable visually, I'd have to dig into some dermatology journal to try to understand it. Mostly I'd fail. But I would pick up the occasional nugget. The most useful of which was this:

Skin is in organ designed to keep what's inside you in, and what's outside you out, and it is incredibly good at it.

There are some chemicals that you can put on your face that do affect the underlying biology of skin. Hydroquinone and Retinol are the two main ones, and hydroquinone is incredibly dangerous and in fact banned in the EU (I think) (check this) and Retinol requires a super serious prescription. Everything else is basically moisturizer. Even the most popular antioxidant, Vitamin C, needs to have a pH below 2.5 to pass into the dermis, which is so acidic as to present its own stresses and to require correction, and on the whole it seems a lot easier to deliver Vitamin C to the dermis by eating it. The idea that you can apply something to the outside to make what's inside healthier just... that's not really how it works.

Except for sunscreen. When it comes to skin ageing, prevention is the only effective cure. And because sunscreen prevents rather than cures skin damage, that makes it easily - as in, by orders of magnitude - the most effective dermatological treatment. People ought to put on, like, SPF 15 every day, same as they brush their teeth every day - it's basic maintenance.

But there again I would say this, being a shill for Big Sunscreen. Any dermatologist / Baz Luhrmann / Mary Schmich can confirm.