Phil Bernheim is in charge of a bulletin board that is part of a national on-line computer network.

Bernheim's job is to help on-line users who are new to computers become comfortable with their machines. But one thing sets him apart from most of his on-line colleagues. Bernheim is 74 years old.

Of course, that doesn't bother his network's users. On SeniorNet, none of the members is younger than 55, and most are 60 to 70.

In the youthful world of computing, SeniorNet is proof that personal computers can be the "empowering tool" for all kinds of people that their manufacturers always claim it can be.

"Age has nothing to do with it," Bernheim said. "In some respects, SeniorNet is like your other information services, just slanted to senior interests."

But somehow, San Francisco-based SeniorNet seems more vital than most other services. While many of its members converse on-line about their travels, some members find SeniorNet is a new lifeline to a world they can no longer travel.

In addition to operating the on-line network, SeniorNet runs 36 sites nationwide and one in Canada where senior citizens can learn to use computers. One of the sites is in a nursing home in Valley City, N.D. Patients who might otherwise talk to few people outside the home can now converse with the service's 4,000 users.

"Communities used to be just geographic, where you lived," Bernheim said. "This is a community of the mind. It's limitless geographically."

SeniorNet is even more amazing when you consider how few senior citizens possess computer skills. Before they can get on-line, they have to face a task many younger people find intimidating: learning to use the computer. And they seem to relish it.

"The network gives seniors a chance to catch up with their grandchildren and great-grandchildren in a skill. And they're such eager beavers; when they learn it, they have it," said Sister Helen Hammack, 70, a nun who is a teacher at the SeniorNet site at the University of San Francisco, where the network started.

The network has other surprises. There's a healthy discussion of the Middle East conflict, decidedly anti-war. There's talk of politics, money and even a little romance.

Mary S. Furlong, the network's president and founder, hopes that word will spread enough to generate more corporate sponsors for new sites. She'd like 100 in the next five years, with 25,000 members.

SeniorNet, a not-for-profit group, is based at 399 Arguello Blvd., San Francisco, Calif. 94118.