Tyler Armstrong of Yorba Linda reached the summit of the 14,505-foot mountain on July 26. It took Tyler and his father 7 hours and 50 minutes to hike from a base camp at 8,500 feet.
His father, Kevin, tells The Orange County Register his son cut his knee and had some altitude sickness but otherwise was fine.
Whitney, located in the Sierra Nevada, is the highest peak in the contiguous United States.
Tyler isn't hanging up his hiking shoes. He's already talking about climbing Mount Kilimanjaro -- at more than 19,000 feet, it's the tallest mountain in Africa.
But because of a climbing age limit, Tyler may have to wait until he's 10.
The Associated Press contributed to this report.