It is possible to get social security disability(SSID) under age 50, but it is not easy. Social security is quite strict regarding physical limitations. In general, at a younger age their rules state that you must either meet a so called Listing or be limited to be able to perform significantly less than sedentary work. Listing of impairments are medical conditions which if met will automatically grant you an allowance.
As an example if you are claimaing disability due to back pain from a herniated disc, if medical evidence from your doctors shows that a cane is medically necessary for walking and you are unable to use your other hand to do sedentary work and still maintain good balance, then your chances for receiving disability are very good. Another situation which would greatly improve your abiblity to get benefits is if due to back pain you cannot sit for very long; as an example, if you need to get up every 5-10 minutes due to back pain. In this situation, there are no jobs you could perform.
The same is true if you have great difficulty walking because of knee pain whether from arthritis or a knee replacement. The key evidence is that according to your doctor’s opinion a cane is medically needed and your unable to perform even sedentary work.
A word of caution, if the state agency for disability questions your doctor’s statement, it may ask you to be examined by a doctor employed by them to actually determine that a cane is medically required.
If you do not meet a listing, there is still a chance that you may be allowed. If the state after examining your medical record determines that you can stand and walk significantly under 2 out of an 8 hour day, your chances for approval are significantly improved.