Could be restless legs, do they feel almost like they are crawling or burning and you can't keep them still? Tonic water might be an idea as it is a natural source of quinine.
Is it one leg or both and are there any other symptoms like swelling or redness? have you had any previous injuries?
I have general inflammatory arthritis and I'm only 30 now. Have some very clicky joints :) Am also hypermobile (joints move more than they should), have secondary fibromyalgia and a vitamin D deficiency (google osteomalacia) all of which contribute to pain. I do get pain when I've active though and much more widespread.
I'm not saying you have any of those but just showing that age doesn't necessarily prevent a problem.
Get your GP to have a look and see if they want to do some X-Rays or blood tests to see if anything is up.
Is there any arthritis in your family or similar conditions or anything like inflammatory bowel or lupus?
Are you sat for long periods? If so then your legs may be stiffening up in reaction and you might need to move more.
Make sure you drink lots of water to minimise any possible fluid retention.
Things like back problems or lack of core stability or weak muscles in thighs can also cause leg pain due to the additional pressure caused so that might be worth considering if you have pain elsewhere. Exercises and equipment such as sitting on a wobble cushion can help with that and osteopathy is an option.