"People have this obsession. They want you to be like you were in 1969. They want you to, because otherwise their youth goes with you. It's very selfish, but it's understandable"

Mick Jagger is describing the public's fixation with keeping him and other celebrities in the same state they remained in during their prime time. Individuals desire them to stay the same since it allows them to hold onto their own youth. It is a self-centered desire, but reasonable as people wish to keep the memories of their own youth alive. Individuals wish to be able to look at their favorite stars and keep in mind the time when they were young and carefree. It is a method for them to stay connected to their own past. Jagger is acknowledging that while this desire is selfish, it is also understandable.