By Gopal Sharma KATHMANDU (Reuters) - An 80-year-old Japanese mountain climber who has had four heart surgeries reached the top of Mount Everest on Thursday becoming the oldest person to conquer the world's highest mountain. Yuichiro Miura, who took the standard southeast ridge route pioneered by Sir Edmund Hillary and Tenzing Norgay 60 years ago, reached the top of the 8,848 meter (29,028 feet) mountain at roughly 9:00 a.m. local time (11pm EDT). He was accompanied by three other Japanese, including his son, and six Nepali sherpas. ...