No cork lasts more than 20 years, older fine wines are re-corked and
use 2 inch corks of highest quality. No synthetic manufacturer
recommends more than 3 years from what I have seen on their websites.
Loss of fruit is a natural consequence of age also.

Cork seepage can be caused by a lot of things, usually upward
temperature excursions. If they feel crumbly at all you should swap
them now; swap all the leakers regardless and maybe touch up to SO2.

Joe