US SENIOR OLYMPICS UNDER WAY IN ST. LOUIS

This week's United States Senior Olympics in St. Louis constitutes ``the largest senior adult athletic event ever,'' says Harris Frank, who heads the sponsoring organization. Some 3,500 competitors came out of qualifying events across the country, in which perhaps 200,000 people participated, Mr. Frank estimates. ``We're coming of age - and these are only our second national games,'' he notes.

There is no ``typical'' competitor, but there are many fascinating ones: Ed Delano, 84, of Davis, Calif., took up cycling after retirement - and at 75 rode his bike 3,100 miles to his 50th college reunion. He won three gold medals in the inaugural 1987 games.

Myron Bishop, 82, of Edwardsville, Ill., is defending his title as the nation's fastest octogenarian, a claim he staked in '87 with gold medals in the 100-meter dash (16.4 seconds) and the 200 (35.21).

Doris Peters, 66, of St. Louis, who never swam competitively until her mid-50s, won two silvers and three bronzes in '87.

The '87 games, also held in St. Louis with some 2,500 competitors, included track and field events, swimming, and cycling (numerous events in each), plus archery, bowling, golf, horseshoes, a 10-kilometer road race, and volleyball. Badminton, softball, and shuffleboard were added this time, with three more sports planned for 1991 in Syracuse, N.Y. - a triathlon, a basketball shooting competition, and a rowing or canoeing sport.

Nearly half of those competing are women, and more than a third are over 70. Medals are awarded to both sexes for each five-year age bracket, starting at 55-59 and continuing to 75-79, then 80-and-over. The latter group is growing so fast, however, that Mr. Frank expects to add catories for 80-84 and 85-89, and 90-and-over next time.