I'm 29 years old, when I cross my left leg over my right my leg falls asleep or I have mild pain in my knee. Is this chiropractic or neurological?

Which leg asleep? The sciatic nerve runs in muscle from the lateral buttocks down the back of the leg, eventually getting more shallow. It supplies most of the nerve sensation of lower leg. If it gets compressed long enough, it will go numb. See when sitting on toilet too long. Same can happen with artery behind knee, as is easily compressed here. Cure for both is same...Change position and shake it out.
Positional YOU. I believe you are compressing the nerve behind your left knee this sensation is tingling, needles, pain, numbness.If your right knee is painful-it is probable that the weight of your left leg is putting pressure on the right knee.If the pain is in your left knee- knee is stretched at an angle over your right knee and straining it, causing pain.Exercise to strengthen your knees.No crossing much.