There are times when extreme youth and freshness are at a premium. This is rarely so in the dance world where stage experience is equated with artistic development. The Joffrey II Dancers, the "junior" company of the Joffrey Ballet, are a refreshing counterexample to this maxim. In recent months they have gained their own special renown because of the presence of Ron Reagan, the president's son, in the troupe. And he danced last night in their concert in Rockville, but without his parents looking on.

In the performance at the Rockville Civic Center, Joffrey II proved that the enthusiasm and winningness of youth go a long way toward an engaging performance. With works shrewdly chosen to play to their strengths, their inexperience showed only in matters of polish, not in bravado.

"Threads from a String of Swing," Daryl Gray's paean to the innocence and fun of the '40s, is a made-to-order Joffrey II vehicle. Here their youth and high spirits paid off. To the sounds of Glenn Miller, three couples danced a stylized Lindy with interspersed balletic lifts and turns.

It is the company's policy to foster choreographic as well as dancing talent and to commission works. These new works most suit the talents of the company. Lance Westergard's "Fantasy at Fiddler's End" is in the mold of Jerome Robbins' "Dances at a Gathering." Cinematic in flow, the movement of the dance features a constant shifting of partners and patterns that melts from one scene to another. Among the dancers cavorting in this platonic dream world, Jodie Gates radiated a lyrical, enigmatic presence and Mona Lisa smile.

The other commisioned work, Leslie-Jane Pessemier's "Forbidden Flights," was a showcase for company talents. Annette Bressie's lyrical presence, Terry Emanuel's elevation gusto and showmanship, Dominique Angel's fire and verve, the romantic tenderness of Jill Davidson and Robert Gardner, and Reagan's style and aplomb were the defining edges of this piece.

Frederick Ashton's exquisite modern classic, "Monotones II," proved an ambitious undertaking. The deceptively quiet simplicity of "Monotones" demands a sophisticated stage presence that radiates even as the dancers stand still. Here the performers were overwhelmed by the choregraphy. These young dancers have not yet captured the subtlety and ease required by this most demanding work.

The program will be repeated tonight at 8 and Sunday at 2.