I used to tan easily. now my legs don't tan at all and haven't for some years. what could be the reason for this?

Could be ...: Oftentimes other areas of the body seem to tan more easily probably mostly because of the angle of the sun shining on the skin. The legs are "harder" to tan because they often shader better than other skin areas.

Medications?: Certain medications may interfere with your ability to tan but this is an unlikely cause and you should exercise caution with excessive tanning.