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He's teasing her because she looks like she's a generation older than him (although of course he's actually hundreds of years older than her).

The 1967 movie (and 1963 novel) The Graduate was about a young college graduate seduced by an older woman named Mrs Robinson. Some of the scenes from that movie are among the most iconic images of the 1960s, and the song "Mrs Robinson" by Simon & Garfunkel was a huge hit in 1968. So, while the joke might seem a bit obscure to younger people today, in 1969, just about anyone would get the reference.