This is a typical Tom Lehrer song, short and in this case rather cynical. If it has a message it is you're only young once, and very briefly, so enjoy your youth while you can. Especially in the bedroom. In September 1967, the man himself told an audience in Oslo this was "another example of pure realism in the popular song."
Two years earlier, the last line was (slightly mis)quoted by a Member of the British Parliament. On November 10, 1965, Robin Maxwell-Hyslop said
"Perhaps the hon. Gentleman will recall Tom Lehrer's song: 'So please remember, when we part in November, I told you so in May'"
Alexander Baron - London, England