Areas in the game are zoned into 6 different zones: 1-19, 20-39, 40-59, 60-79, 80-98, 99+. It means a player in the 20-39 zone will not share the same area as a guardian in the 1-19 zone. This allows lower level players to not have to compete for kills with higher level players that are outside their zone's levels. Public Events & Social areas will still be shared by players of all levels.
Chests also start showing up, once you reach level 20, and additional types of chests are introduced with each level zone you reach. They are as follows:
20-39: Small Chest, Medium Chest
40-59: Large Chest, Small Material Cache, Medium Material Cache, Common Engram Cache, Uncommon Engram Cache
60-79: Large Material Cache, Rare Engram Cache, Single Core Box