About 20 years sometime this year courtesy of a secondhand Specialized Street Stomper which I believe is still owned by my (now) ex father-in-law. It would have been a year or two sooner if I hadn't spent the last years of the eighties paying out 90% of my wages each month on a mortgage.
I could claim 30 years ago if you count the scrapper I built up in my early teens for riding the local woods and spent two terms of metalwork lessons making suspension forks for.
EDIT
Just remembered the first event I entered. The Isle of Wight Scouts had orgainsed an inter troop kiddies cyclo-cross for us all. I can remember riding a borrowed bike and the course being very muddy.