Exclusive: Youngest to Climb Mt. Everest Was 'Exhausted' at the Top

When the youngest American to ever climb Mt. Everest reached the mountain's summit, she couldn't believe it, she told "Good Morning America" anchor Diane Sawyer exclusively on Friday.

"I really couldn't believe that I was finally there," Samantha Larson told Sawyer. "I was really exhausted."

The 18-year-old Larson, from Long beach Calif., made headlines around the world this week by climbing to the top of the nearly 30,000 foot mountain, the world's tallest. With the climb, she became the youngest member of an elite group of just 200 people who have scaled the tallest peaks on all seven continents.

"Our summit day was really nice...there weren't many clouds. There were great views," Samantha said. "I had different levels of fear at different times, but I thought we were pretty safe."

The teen climbed the mountain with her father, David Larson.

"We evaluated and measured risk" when deciding whether his daughter could or should climb the mountain, her father said. "Samantha's done her homework. She's trained on mountains in seven continents."

Although questions have existed about younger people's lung capacity in higher altitudes, Larson said his daughter's ascent proved that young lungs can acclimate.

"I got the summit a little before my dad and so I met him when were coming back down," Samantha told Sawyer. "And I gave him just a big hug."

Once you get to the mountain top, according to Larson, you can enjoy the views, but any movement on the climb back down is tough.

"It's amazing when you're [at the top]. You can relax," he said. "As soon as you start moving, you're short of breath."

As for Samantha, she says that just because she scaled Mt. Everest doesn't' mean she's done climbing.

"There's plenty left to climb," she said.