OP, this is one of those issues you'll have to trust yourself on. The short answer is, yes, even those who tan can get skin cancer as can dark skinned people. However, moles and other warning signs aren't as noticeable on darker skin which is why Asian American and African American melanoma patients have a greater tendency than Caucasians to present with advanced disease at time of diagnosis. To be fair, the risk is much higher for light skinned people, but tanning or dark skin is not an immunity by any stretch of the imagination.
Skin cancer is the most commonly diagnosed cancer in the world and incidences of it are on the increase. One in five Americans will develop skin cancer in the course of a lifetime. (The statistics include the fact that those born in the 1990's have a three times higher lifetime risk than those born in the 1960's.) There's also a misconception that skin cancer is easily treated by removing a mole or some such, so not really anything to be worried about. While if caught early the common types of skin cancer are highly treatable it is still a serious and potentially fatal disease. Squamous cell carcinoma (SCC) is the second most common form of skin cancer. An estimated 700,000 cases are diagnosed each year in the US, resulting in approximately 2,500 deaths.
If all that isn't enough to wear sunscreen, how about the idea that 90 percent of the visible changes commonly attributed to aging are caused by the sun. A bottle of anti-aging cream will do nothing but moisturize damaged skin. A bottle of sunscreen will prevent many of the signs of aging from happening.
All that said, the risks associated with the sun are still a mystery to many people. And the idea that the sun can be harmful is still a hard concept for many people to wrap their minds around. Add to that the fact that sunscreen is a bother to apply and you have to pay for it, and it's not hard to see why it's easier to do without.
The only thing I would contest in your statement is the idea that "most of our skin damage occurs when we are children." This isn't actually true. Contrary to popular belief, 80 percent of a person's lifetime sun exposure is not acquired before age 18; only about 23 percent of lifetime exposure occurs by age 18.
Here's a little graph from the Skin Cancer Foundation:
Lifetime UV Exposure in the United States
Ages Average = Accumulated Exposure*
1-18 = 22.73 percent
19-40 = 46.53 percent
41-59 = 73.7 percent
60-78 = 100 percent
*Based on a 78 year lifespan
For more information try
www.skincancer.org or
www.canadianskincancerfoundation.com