This Friday night, principal dancer Kyra Nichols will retire from New York City Ballet, where she has danced for 34 years. She’s now in her late 40s; her retirement can’t be called premature. Yet it’s not overdue by any means. Today, Ms. Nichols is not the dancer she was 25 years ago — she is better. One of the lessons to be learned from her career is how smart it is for a company to let a dancer stay onstage well into middle age, provided her technique remains respectable and her repertory is chosen wisely. A company begins to decline when this kind of continuity is not permitted. Ms. Nichols has said in interviews that her dancing really blossomed after the birth of her two children in the 1990s. Certainly in the last decade, she has interested and impressed me as she never had before.

Ms. Nichols’s retirement from NYCB also severs one of the last direct links to company founder George Balanchine. When Ms. Nichols leaves, principal dancer Darci Kistler will be the only remaining ballerina to have received the master’s direct tutelage. Balanchine promoted Ms. Kistler to principal dancer in 1979, when she was 20. It wasn’t particularly unusual for him to promote dancers to the top rank when they were this young, but it certainly speaks of his high regard for her. Balanchine’s interest in Ms. Nichols’s development was facilitated by Jacques d’Amboise, who was then a senior principal dancer. Mr. d’Amboise organized small groups of dancers and took them on tour, and he fostered Ms. Nichols’s career, making sure that Balanchine took stock of her. Jerome Robbins, too, was a catalyst in Ms. Nichols’s progress within the company.

Ms. Nichols arrived at NYCB in 1974 with an auspicious pedigree. Her mother, Sally Streets, now a prominent Bay Area ballet teacher, danced with the company in the 1950s. At that time, Balanchine’s choreography and teaching was very concerned with training Americans to be comfortable with classicism: classical positions, classical precision. This was the departure point for his own neo-classicism. Ms. Nichols began her ballet training with her mother, and was, to some extent, a product of the emphases Balanchine developed during her mother’s era. Yet by the time Ms. Nichols began to rise through the company ranks, Balanchine’s focus had shifted, and his aesthetic was crystallized in the roles he made for Suzanne Farrell, who had returned to NYCB in 1975 after six years away.

Although Ms. Nichols had always shared ballerina roles with Ms. Farrell, it was only after Balanchine died in 1983 that she began to regularly dance many of the roles that Balanchine had created especially to showcase Ms. Farrell’s style. For me this wasn’t, initially, a happy succession: In the Farrell repertory, Ms. Nichols could seem clipped and perfunctory — physically, musically, stylistically. She did not throw caution to the wind with the profligate manner Ms. Farrell did. She didn’t, and perhaps at that time couldn’t, take those kinds of liberties.

It may be that Ms. Nichols’s more cautious temperament, as well as her rock solid classical training and aplomb, have allowed her to last as long as she has. A parallel could be drawn to the world of opera, where the more conservative Joan Sutherland sang well past age 60, whereas the vocally risky Maria Callas and Beverly Sills suffered in longevity.

At a slow and steady pace, Ms. Nichols succeeded in making the Farrell niche of the NYCB repertory her own; she adapted herself to dance expressions that now surpassed proficient technique, and she has coalesced into a fully theatrical as well as kinetic animal. Two months ago, at NYCB’s spring season gala opening, she danced Balanchine’s “Pavane,” a solo in which she accompanies herself with a bolt of chiffon. We saw once more the way she could now manipulate theatrical flamboyance that was not originally second nature to her. She posed, she reveled in the scarf as adornment, as containment, as confidante, and as partner. She had become warmer, freer, and more imaginative than ever before. Ms. Nichols is a dancer who has never stopped learning, and on Friday we’ll look forward to see the summing up of her three decades of onstage achievement.