Can you thicken the cartilage discs in your spine by stretching it?

No. Degenerative disc disease (ddd) is a progressive change of the spine that includes loss of disc water content and disc height. We all get ddd as we age, and as yet, there is no treatment to prevent it (despite what people will try to sell you). The best option to maintain spine health is to not smoke, maintain a healthy overall lifestyle which includes a regular exercise program.