The number of once-a-year moviegoers are on the rise and--contrary to what one might think--that's good news to the National Assn. of Theater Owners. Results of recent survey, prepared by the Motion Picture Assn. of America, show that the number of people who go to the movies at least once a year increased 8.5% over 1986. The largest increase in moviegoing came from those 60 or older, up 50%; the 50-59 age group went up 26.3%; and a rise of 18.6% was recorded for those aged 40-49. With the youth-movie craze of the last few years on the wane, so are youthful audiences--attendance was down 1.4% from 1986 for those under 30.