Could be a natural with age and as stated above, this differs with individuals so what is "normal".
Do you have a history of pain or stiffness, an injury earlier in life perhaps?
If it is disturbing your sleep or making your normal routine very hard or impossible then I would have a chat with your GP.
As to a specific condition, I wouldn't feel qualified to make suggestions. Possibly look at changing your mattress, it did my dad the world of good as he was aching a lot after a long sleep and it was put down to a very soft mattress