This is a typical case of osteochondroma, typical clinical presentation (age of patient, painless lump), typical location (femur is the most affected bone), typical appearance (pedunculated bony outgrowth that directed away from the joint).

Osteochondromas are the most common bone tumors in children, may be solitary or multiple. The true prevalence of solitary osteochondromas is not known, because many asymptomatic lesions go undiagnosed.  A painless skeletal swelling or a slowly growing mass is the usual mode of presentation. 

Most osteochondromas, solitary or multiple, arise from tubular bones and are metaphyseal in location.