Like everything else, PV modules degrade with time. The aging or derating factor is usually covered by the manufacturer performance warranty. Most major brands, listed in the table below guarantee a maximum of 20% de-rating over 25 years, with different milestones during that period. Some manufacturers include the initial LID at year 1 with values varying from 2.5% to 3.5%.
In reality, the PV de-rating would be lower than the guaranteed limit. Solar system design can use 15%.