Question
Would you happen to know the statistics on the number of people in the U.S. who have back problems between ages 18 and 35, or a figure like it? It seems more and more I am coming across people who to me seem rather young, and yet they've had back surgery for troubles they've had, or feel they might need surgery.

Answer
There was a study done years ago, I'm not exactly sure by whom, that showed of MRI's done on people, 80% had a disc issue, with NO symptoms.  The number of people with back pain, from the aCA website:
A few interesting facts about back pain:
One-half of all working Americans admit to having back pain symptoms each year.2
Back pain is one of the most common reasons for missed work.  In fact, back pain is the second most common reason for visits to the doctor抯 office, outnumbered only by upper-respiratory infections.
Most cases of back pain are mechanical or non-organic梞eaning they are not caused by serious conditions, such as inflammatory arthritis, infection, fracture or cancer.
Americans spend at least $50 billion each year on back pain梐nd that抯 just for the more easily identified costs.3
Experts estimate that as many as 80% of the population will experience a back problem at some time in our lives.4