First off, solar panels have a lifespan of between 25 and 40 years. It's a pretty big range that depends on how well you take care of them, and the brand of solar panel you purchase.
Second, most people do experience minor drops in efficiency over the years. The usual estimate is that you'll experience a 3% efficiency loss in the first year, followed by a 1% loss each additional year. Make sure to talk with the installation companies you're involved with about solar panel warranties and whether they cover efficiency loss.