If you have spina bifida occulta at age 16, could it get worse with age, or is it very stable?

Don't worry. This is a very common x-ray finding noted in about 22% of the population and is known as an incidental finding with no bearing on any stability issues and is asymptomatic meaning it does not cause pain. It also does not change over time. It can be associated with spondylolithesis and some transitional vertebra or base of spine anatomy variants that may alter spinal mechanics and cause pain.
Spina Bifida. Spine bifida occulta is present when there is a cleft between the left and right components of the spine usually at the L5 level. Most of the time there are no symptoms but this process can be associated with alignment problems of the spine and coupled with expected degeneration of the spine, can become unstable or symptomatic later in life.