The ages of the victims in the San Diego-area mass suicide and the affluent neighborhood in which it took place fit the profile of modern cult activity, an expert on new religious movements said Wednesday.

Ronald E. Enroth, a sociology professor at Westmont College in Santa Barbara, Calif., also said he believed the deaths _ if they are ultimately linked to a religious group _ would be the largest such suicide in U.S. history.

Thirty-nine young men and women between the ages of 18 and 24 were discovered dead Thursday afternoon in a mansion in the well-to-do San Diego suburb of Rancho Santa Fe, an area replete with tennis courts, swimming pools and corporate leaders' homes.

``That age bracket is precisely the target population for cults. Eighteen to 26 is the group that most of the new organizations are targeting,'' Enroth said. ``And if it was in an upscale neighborhood, the new groups have been targeting the upper middle class.''

Before 1950, Enroth said, such cults as the Father Divine movement and the Sweet Daddy Grace movement targeted disenfranchised Americans. But in recent years, he said, religious groups have been recruiting more affluent members.

``This does not appear to be people on the margins of society,'' Enroth said.

Initial reports, he said, hint at what is a typical cause of mass suicides.

``To put it in very unprofessional, unacademic language, it looks as if somebody snapped _ probably the leader,'' Enroth said. ``Some emotional trigger occurred and these people followed suit.''

Cult experts say leaders of religious groups often consider themselves messiahs _ the living embodiment of doomsday prophecy _ and tailor their actions accordingly.

Enroth, author of ``The Lure of the Cults and New Religions,'' received the 1982 Leo J. Ryan Commemorative Award, named for the California congressman shot on an airport tarmac while in Guyana investigating the Rev. Jim Jones' Peoples Temple in 1978.

Hours after Ryan was killed, more than 900 members of Jones' group died, most after following his instruction to drink cyanide-spiked grape punch.

Enroth said many cults are entirely unknown before their activity bubbles over into violent incidents.

``I know of groups that nobody's heard of that have the potential for violence,'' he said. ``Who heard of Jim Jones before Jonestown? Who heard of Koresh before Waco? There are lots of groups out there who have never been written up who have the potential to become violent.''