You are NEVER too old to begging your yoga practice!
Can I practice yoga with injury?
Yes. However, it depend what nature is your specific injury, so the best thing is to get clearance from your doctor first, and than find a good yoga teacher who is knowledgeable in working with injuries.
How many times a week do I need to practice yoga to get supple?
Well, that depends on your initial suppleness, but on average it is recommended to practice 2-3 times a week to see results.