Reversing the trend in recent years, more adults and fewer teen-agers went to the movies last year, according to the Motion Picture Association of America.

The findings are contained in a speech MPAA President Jack Valenti was to make today at ShoWest '88, a convention of theater owners in Las Vegas.

According to the prepared text, the over-40 age group accounted for 20 percent of the record $4 billion-plus in gross receipts last year, while the teen-age audience accounted for 25 percent. In 1984, when the previous box office record was set, the over-40 figure was 15 percent and the teen-age audience 32 percent.

"There is little doubt," Valenti's speech concludes, "that for the foreseeable future, the image of teen-ager as the king of the movie hill has developed chinks of clay." No longer can the studios, writers, producers and distribution companies "blindly count on growing legions of teen-agers to populate the theaters. They must reach (up or down, depending on how one gauges these things) for the older prospective movie patron."

While delivering MPAA's customary industry cheer, Valenti's speech also notes some less hopeful news: In 1987 the average cost of producing a major movie increased to $20 million, up 15 percent from 1986 and 113 percent from 1980.

After marketing and advertising costs are figured in, this means a $20 million movie will have to gross $29 million to break even. And while the total of theater screens rose 3.5 percent over 1986, 1987 also saw a 5 percent rise in the average ticket price, to $3.90.