Widening of upper mediastinum is due UNFOLDING OF AORTIC ARCH.

With advancing age, aorta tends to elongate and dilate, resulting in the so called unfolding of aortic arch.The aortic knob becomes larger and higher.The descending aorta becomes more tortuous, hence convexity increases which may appear like an aneurysmic dilatation. 

The upper mediastinal widening in the infant is due thymic shadow. (normal)