Internet research shows material life for carbon fiber & resins (lifetime), fiberglass (50 years), and plastics / foams (1000 years). High stress applications for these materials include automotive, boats, mountain bike frames, etc. Even though water skis are stressed during use, they are used seasonally, often not used because of upgrading, mostly used in fresh water, and kept inside out of the elements. Any ski, if well cared for, should last a long time. To prove otherwise would require extensive testing of identical samples, using mechanical means in controlled conditions, and statistical analysis of the results.