The Flyby special trait states, for example:
The peryton doesn't provoke an opportunity attack when it flies out of an enemy's reach (MM 251).
The name of the trait implies that an creature approaches an enemy target while in flight, attacks, and then continues past out of the enemy's reach without provoking (thus flying by). The description, however, doesn't appear to restrict the advantage to this particular sequence.
Would you apply this advantage if a creature starts its turn on the ground adjacent to an enemy and uses its fly speed to move away (thus flying out of an enemy's reach)?
For the purposes of this question, it would probably be helpful to ignore the mechanics necessary for a winged creature to launch itself into the air, and think more in terms of magical (i.e. instant take off) flight.