As popular as Boone was, there is not a wealth of imagery of him. As you see here, a few illustrations and paintings exist, most of which show him in his later life. The poplular 3/4 view pose, lack of expression, frown included, was the typical representation of subjects during that time period.

Perhaps old Boone just never slowed down long enough until his later years to be painted. He was busy creating his legacy.