I've had more than one person look at me incredulously when they ask me how long I've been cycling, and I tell them "since I was 5 years old." I will be 55 in July. Aside from a couple of weeks of downtime once when I had a bicycle stolen and therefore had to find a suitable replacement, I have been cycling since. I don't really remember my first real road bike, I think it was some Sears Freestyle model, but the first road bike I raced was an 80s Raleigh sport-touring model, don't remember which. I remember removing all the non-essentials (kickstand, reflectors, safety levers) and moving the shifters from the stem to the downtube. I was able to hold my own on club rides with guys riding their DeRosa and Colnago bikes.
Probably not the longest, but 50 years of almost uninterrupted cycling ain't too bad.