There are many factors that need to be considered when estimating the length of time solar panels will last. These include:

• The year the solar panels were produced
• The quality of the panel (higher quality panels tend to last longer)
• Time panel gets direct sun each day

The EU Energy Institute estimates that 90% of panels purchased in the last ten years will last 30 years. Panels manufactured in 2013 or later should last 40 years. They also noted that about 90% of all panels will outlast the estimated lifespan – albeit with a drop in performance.

It should be noted that while the lifespan of the solar panels are getting longer, inverters still have a life span of approximately 15 years.
Sources:
http://www.ul.com/global/documents/newscience/journal/sustainableenergy/New_Science_SE_Journal_Issue_3.pdf
http://www.solar-rating.org/facts/system_ratings.html
http://www.ee.institute.org