The second group was not told to avoid sunscreen, but rather to use it as they wished.
The study also assessed the potential impact of dietary antioxidants on skin quality. Each group of participants was further divided into half, in which one sub-group was given 30 milligrams of beta-carotene to take on a daily basis, while the other was given a sugar pill (placebo).
Using a process called "microtopography," the team then analyzed skin changes such as shifts in texture and roughness, by looking at the status of each participant's left hand.
The result: Beta-carotene was found to have no appreciable impact on skin aging among those taking the supplement.
But after 4.5 years, regardless of age or gender, those using sunscreen every day were found to have experienced much less skin aging compared with those who did not slather daily.
Over the course of the study period, 24 percent less skin aging was found among the dedicated daily sunscreen group compared to those who used sunscreen only when they wished (presumably less frequently).
That said, Green stressed that the observed skin protection benefit of sunscreen requires some attention to detail.
"This research applies to SPF 15 sunscreen," she said, "and, crucially, applying it correctly on a regular basis. Sunscreen must be applied thoroughly, with a thick coating, and must be reapplied every few hours in case it's washed or sweated off."
"And of course, along with seeking shade and wearing clothing over, sunscreen is a mainstay of sun protection," Green added. "It prevents sunburn in the short-term and skin cancer in the long-term."
And what of non-sunscreen cosmetic products, used by millions of women, that claim to have bundled SPF protection into the mix?
Green cautioned that there is no replacement for true sunscreen.
"Moisturizers with UV protection give brief sun protection," she acknowledged, "but sunscreens last much longer on the skin and provide more durable protection."
Dr. Beth McLellan, an assistant professor at the Ronald O. Perelman Department of Dermatology at the Joan H. Tisch Center for Women's Health at NYU Langone Medical Center in New York City, said the findings add more evidence that sunscreen works.
"We already know that sunscreen can prevent skin cancer and this study shows that regular use of sunscreen can also help prevent aging -- all the more reason to incorporate sunscreen into one's daily skincare regimen," she said.