This song is from The Great Gatsby film (my opinion about novel is here).
This song, how I think, tell us about one famous, rich and old (not very old) women.
First, this women «seen the world», and «done it all», that means, that she has anything and she, may be, tried anything, what can try or can has anyone at our planet.
Now she «has her cake now». That means, that she can just eat cake and do nothing. Because, she do not need to do something, she has everything.
But, she has everything and she wants to feel. She wants to feel love, she wants to feel some man (not specified man). She wants to be loved by some man. But, also, she has large number of ages, and she is not sure about be loved by this abstract man and we can see it on chorus: «will you still love me, when I am no longer young and beautiful». And, how I think, she is not «young and beautiful» now.
May be, idea of this song is woman can has anything, but she always need to be loved.
I can not write very much on english and I still do not have last version of understanding of idea of this song, so, at this time, it is enough for my opinion.