The most recent issue of the Annals of Internal Medicine contains an article from Australian researchers in which nearly a 1000 adults under age 55 participated in a study to apply daily sunscreen to sun exposed areas (hands, arms, face, etc.). After 5 years of following these study participants, those who had followed the sunscreen regimen experienced 24% less skin aging as measured by skin texture as compare to those who only used the sunscreen occasionally.
Bottom line: Daily use of sunscreen does slow the aging process down of exposed skin areas. The key here is daily use, not just when you think you might get sun exposure. Use SPF 15 or higher.