VENTURA, Calif. — Martin Knowlton, who co-founded the Elderhostel educational tour program for senior citizens at a time when the concept of lifelong learning was revolutionary, has died. He was 88.

Knowlton died of natural causes on Thursday at a nursing home in Ventura, Calif., Elderhostel representatives announced Friday.

Boston-based Elderhostel provides packaged tours to more than 90 countries combining lectures and activities. Its programs offer everything from Amazon boat trips to opera studies.

Knowlton was director of the American Youth Hostel at the University of New Hampshire in 1975 when he and David Bianco conceived the idea of Elderhostel. An Elderhostel statement said it was inspired by youth hostels and by Scandinavian adult education folk schools that Knowlton had studied while backpacking through Europe.

At the time, the idea that senior citizens could or would like to continue their education was considered revolutionary.

Elderhostel said it now attracts more than 160,000 participants annually with nearly 8,000 tour packages.

Knowlton left his position as executive director in 1977 after Elderhostel became a not-for-profit corporation, writing that he was damaging the organization by resisting administrative changes. However, he was appointed to the board of directors and remained on the board until 1984, Elderhostel spokeswoman Despina Gakopoulos said.