It is not very common at this age. I would consider thinking of it as more of a conditioning issue. I suppose it is a getting back into shape issue more than anything. As we grown older the chances of bad vasculature in the legs increases. It could represent electrolyte problems, these include issues with sodium, potassium and calcium. This is unlikely if you eat a balanced diet and if you are otherwise healthy. I would advise you to ease into exercise. Start with smaller, shorter walks and increase them slowly over time. Make sure you are hydrated and eat plenty of fruits and vegetables. If you have a strong family history of high cholesterol or a history of young family members with heart disease, you may be in a position to ask your doctor for a check up. If not, it is just a question of getting your body tuned to exertion and exercise.