Which opera deals with youthful love, the fading of youth, the passing of time?

The final trio.  Anke Vondung as Octavian.  Maki Mori as Sophie.  Anne Schwanewilms as Marschallin.  Directed by Uwe Eric Laufenberg. The orchestra is the Dresdenstaatskappele.

From Wikipedia:
The Marschallin, Sophie, and Octavian are left alone. The Marschallin recognizes that the day she so feared has come, as Octavian hesitates between the two women (Trio: Marie Theres'! / Hab' mir's gelobt). In the emotional climax of the opera, the Marschallin gracefully releases Octavian, encouraging him to follow his heart and love Sophie. She then withdraws elegantly to the next room to talk with Faninal. As soon as she is gone, Sophie and Octavian run to each other's arms. Faninal and the Marschallin return to find the lovers locked in an embrace. After a few bittersweet glances to her lost lover, the Princess departs with Faninal. Sophie and Octavian follow after another brief but ecstatic love duet (Ist ein Traum / Spür' nur dich), and the opera ends with little Mohammed running in to retrieve Sophie's dropped handkerchief, and racing out again after the departing nobility.

This production of Strauss: Rosenkavalier available on Amazon.

From Wikipedia:

Der Rosenkavalier (Op. 59) (The Knight of the Rose) is a comic opera in three acts by Richard Strauss ...
The opera has four main characters: the aristocratic Marschallin, her very young lover Octavian Rofrano, a part sung by a woman, her coarse, philandering country cousin Baron Ochs auf Lerchenau, and his young prospective fiancée Sophie von Faninal, the lovely daughter of a rich Viennese bourgeois. Baron Ochs, having arranged with Sophie's father Faninal to combine his noble rank with Faninal's money by marrying Sophie, asks the Marschallin to suggest an appropriate young man to be his Knight of the Rose, who will present a silver rose to Sophie on his behalf as a traditional symbol of courtship. She recommends Octavian. When Octavian delivers the rose, he and Sophie fall in love on sight, and must figure out how to prevent Baron Ochs from marrying Sophie. They accomplish this in a comedy of errors that is smoothed over with the help of the Marschallin.^[4]

But while a comic opera, Der Rosenkavalier also operates at a deeper level. Conscious of the difference in age between herself and Octavian, the Marschallin muses in bitter-sweet fashion over the passing of time, growing old and men's inconstancy, and it is hard to escape the conclusion that her nomination of him as Knight of the Rose and his and Sophie's subsequent love are no accident - realising that she will inevitably lose Octavian sooner or later, she has chosen to set him free.