The two most intriguing, and certainly contrasting, dance events this weekend cost nary a farthing.

It's a tired cliche that dancers have to be young, reed-thin and malleable to reach an audience. The DANCERS OF THE THIRD AGE, one arm of the Dance Exchange operation, are one dynamite performing ensemble, and their youngest member is a mere 60 years of age. They've toured from Florida to Boston and performed numerous informal and concert works throughout the Washington area.

This Friday at the Museum of American History, the eight-member troupe (plus several members of the Dance Exchange Performance Company), will present a dance warm-up, perform selected numbers from a 15-piece repertoire that includes a unique interpretation of "Swan Lake," and invite the audience to get in on the action.

It's no secret that all those nifty gyrations attributed to "Flashdance" star Jennifer Beals were in fact performed by an uncredited double. That juiced-up dervish, Frenchwoman MARINE JAHAN, will be dancing and signing autographs at Fairfax's Fair Oaks Mall this Saturday at 2. Jahan will be preceded by a local troupe, the Feet First Jazz Dancers. To the rhythms of Michael Jackson, David Bowie and Frank Stallone (of "Staying Alive" fame), these nine performers will present a choreographed fashion show with gymnastic and jazz routines.

DANCERS OF THE THIRD AGE -- Friday at noon at Carmichael Auditorium, Museum of American History, Constitution Avenue between 12th and 14th streets NW; free; call 783-8900.

MARINE JAHAN & FEET FIRST JAZZ DANCERS -- Saturday at 2 at Center Court stage of Fair Oaks Mall, intersection of U.S. 50 and I-66, Fairfax; free; call 347-3545 or 656-9076.