In a way, you're both right. Horse racing can be a dangerous sport, right up there with motor sports, diving, and rugby. In addition to the risk of falling from a fast-moving horse, there's the stress that riding puts on your back and neck over time. 

A recent study showed that jockeys had more low back and neck problems than people the same age who didn't ride. The study didn't look at whether older or younger jockeys had more injuries. However, it did show that jockeys over 35 had more back and neck pain than their younger counterparts. This is probably because the older jockeys had been riding an average of 24 years while the younger jockeys had only been riding an average of four to 12 years. 

The longer you ride, the more the sport takes its toll on your body. In the meantime, ride carefully, and give your back periods of rest.