For me, it was July 1964, LHR-BSL. But I can't claim to remember anything as I was only 5½ weeks old.
My Junior Jet Club logbook is, fortunately, safe at home. Unfortunately, it only starts in 1974. I think I could probably reconstruct much of my previous flying from the collection of ticket receipts, though, which I have continued to maintain even though recently they have been far less glamorous than they used to be.