For 21 years, Edward Evans investigated prospective CIA employes. For 20 years, James Casey was a government attorney. Christian Mendehall was a professional actor and a singing waiter. Joseph O'Hare taught school.

Now -- in their second careers -- they are all Catholic priests.

Their stories are not unusual. Over the past 15 years, the number of men entering seminaries after they are 25 has risen dramatically. Some seminaries report that up to 30 percent of their current enrollments are "delayed vocations" or "second career" seminarians, those over the traditional limit of 250.

Other seminaries designed especially for men over 25 are filled to capacity or, in one case, forced to turn men away.

Just over a decade ago many of these men would have had trouble getting into seminaries, according to the Rev. William Ferree, director of the Second Career Vocation program in Dayton, Ohio. But because of declining seminary enrollments -- from 34,000 in 1969 to 14,000 in 1979 -- the church is taking a second look at older men who previously would have required dispensations to be admitted.

And, according to seminary rectors, pastors and counselors, this new breed is able to empathize more closely with their parishioners and are far less likely to leave seminary than their college-age counterparts.

"Older men have always wanted to join the priesthood," Ferree said, "and that was the accepted practice until the mid-19th century."

According to Ferree's research, St. Edward the Confessor became a priest in the 11th century at age 50. St. Paul the Simple entered religious life in the 4th century when he was 60, and in the 16th century, St. Ignatius Loyola joined the priesthood at 43.

"It's quite clear that beginning in the 19th century there was an identification of the vocation with the young," said Ferree. This "youth philosophy" is traced to the French teaching orders that encouraged young boys to enter seminaries "before they were contaminated" by the opposite sex, he said.

Because many teaching orders originated in France, the practice spread, until about 10 or 15 years ago, said Ferree, when gray-haired men began showing up in seminaries once again.

The growing number of older seminarians has pushed the age of the average seminarian from 23.1 in 1970 to 27.1 in 1976, according to the summer 1980 issue of Review of Religious Research.

The trend may be temporary, according to the Rev. Gordon Henderson at the Center for Applied Research in the Apostolate. "Following the changes of the second Vatican Council, men delayed their vocations until things settled down." He thinks the number of older seminarians will taper off in a few years.

Evans, the former CIA employe, was ordained last month at age 48. "I felt I had been called by the Lord all along," he said. But instead of entering a seminary, he chose at first to remain very active in his church. His feelings intensified, and he quit his $31,000-a-year job.He is now an assistant pastor at St. Ann's Church in the District.

Casey, who was 57 when he was ordained 11 years ago, said he wanted to be a priest ever since he was an altar boy. But then his father died and he had to support his family so his dreams were pushed aside. Casey, now pastor of St. Michael's Church in Baden, Md., said he regrets not entering the seminary 20 years ago.

O'Hare also always wanted to be a priest. But first he wanted to see what it was like to pay bills, balance a checkbook and support himself. He taught for three years in Philadelphia parochial schools and was ordained last year at 29. Now he is assistant pastor at St. Gabriel's Church.

Mendenhall loved acting and dancing, but it wasn't enough for a lifetime for the Mormon convert. After being ordained last year at age 31, Mendenhall is now assistant pastor at St. Jane Frances de Chantal Church in Bethesda.

None of the priests interviewed had been married, but several said they attended the seminary with widowers, some of whom had children and grandchildren. According to church law, widowers may become priests as long as they have no dependent children.

The most frequent problem older seminarians said they faced was learning how to study again, sometimes after a lapse of 30 years. Other said they had problems adjusting to the structured environment, even though seminaries designed for them give them more flexibility than the traditional ones.

"Celibacy is a sacrifice," said the Rev. George Griffin, who recalls having a date the same day he began making arrangements to join the seminary. "But many a night after a rough marriage counseling session I go to bed thanking the good Lord for celibacy. There's a lot of pain out there." Griffin, now associate pastor at Good Shepherd Church in Alexandria, was ordained seven years ago at age 35.

Most said they didn't mind giving up material possessions, that it made life easier.

But the years, even decade the priests spent working as bureaucrats, teachers, policemen, and the like were learning experiecnes that now help them help their parishioners cope with day-to-day life.

"I know what it's like to be chained to a desk all day, to get stuck in traffic jams, and to pay bills," said Griffin, a former accountant. "It's an experience I can draw on."

The Rev. Michael Blackwell, who was in the Navy until 1970 when he entered the seminary, said, "My experience has been absolutely invaluable, I would be far less effective without it." Blackwell, whose Navy years included a tour of duty in Vietnam and service as a White House social aide, is now with St. Michael's Church in Silver Spring.