The lower boundary of outer space has been adjusted numerous times, but a final value was set at what is known as the Karman line, which is fixed at 62 miles above sea level, according to Universe Today. This boundary was defined early in the Space Age.
Prior to the adoption of the Karman line, the boundary of space had been set at 50 miles, according to Universe Today, and pilots who exceeded this boundary were eligible to wear astronaut's wings. The modern definition takes physical forces into account. Above the Karman line, a flying object derives more lift from orbital velocity than it could from the flow of air over its flight surfaces.