Kalos Route 22 is accessible very early in the game, but the player is not required to visit until the end of the game. Due to the fact it is accessible very early in the story, Pokémon levels are balanced for that point. I believe the games encourage the player to visit Route 22 when it first opens up, but the Bulbapedia guide I based the chart off of has the player wait until the end of the game to enter the area.
Multiple boss encounters are battled during the players raid on Lysandre Labs and the Team Flare Secret HQ. In addition, two boss encounters are battled at the Tower of Mastery.
Much like Pokémon Black and White, the Elite Four can be battled in any order, and their levels are balanced to accomodate this.
Observations
Boss trainers tend to stick pretty close to the level curve for regular trainers, although they tend to be a bit closer than in Black and White.