Useful life (also termed service life or operational life) is defined as the life achieved by the capacitor without exceeding a specified failure rate. Total failure or failure due parametric variation is considered to constitute the end of the useful life.
Depending on the circuit design, device failure due to parametric variation does not necessarily imply equipment failure. This means that the actual life of a capacitor may be longer than the specified useful life. Data on useful life has been obtained from experience gained in the field and from accelerated tests.
The useful life can be prolonged by operating the capacitor at loads below the rated values (e.g. lower operating voltage, current or ambient temperature) and by appropriate cooling measures. In addition to the standard type series, EPCOS types are available with useful life ratings specially matched to customer specifications.
Calculating the Useful Life of Capacitors - [Link]