I had my first lumbar fusion at age 10 ('68), then another at higher levels two years later with instrumentation (this additional surgery was anticipated, as I have scoliosis...it wasn't so much because of any additional stress on the vertebrae above the fusion). This served me well for 20 years, and probably would've went longer if a group of surgeons hadn't seen all the hardware and dollar signs flashing in front of their eyes.
In the interim, I was quite active...running three miles a day and working in a furniture factory. Ahhhh....but I was young then.