In Terraria Leveled each biome eg. desert or forest, contains different levelled mobs. Upon starting a new world you will notice levels of biomes may change during night. also upon entering hard mode new levels appear on each biome.
Forests contain mobs around level 2 during the day and 5 during night pre-hardmode such as your classic green slime or zombie. During hardmode mobs in the forest can range from 48 during the day and 49 during night.
Going bellow the forest biome you encounter the cavern which contains mobs around level 10 pre-hardmode and around level 53 in hardmode.
Deserts contain mobs between the level of 12 pre-hardmode and 56 in hardmode.
Snow biomes contains two separate biomes. the surface snow biome and the underground ice biome. the surface snow biome also possess different levels during day and night. During the day, surface biome contains level 6 mobs pre-hardmode and level 60 during hardmode.
During night the snow biome features level 8 mobs pre-hardmode and around level 55 mobs during hardmode.
Underground ice biome features mobs around level 16 pre-hardmode and around level 59 during hardmode.
Corruption Biomes once again have a surface and underground section. surface corruption contains mobs around level 24 pre-hardmode and around 57 during hardmode.
Underground corruption contains mobs around level 60. note
underground corruption only appears in hardmode.
Crimson follows the same path as corruption with a surface and underground that both possess the same levels. underground crimson follows the same rules as corruption.
Jungle biomes posses a surface and underground component. surface jungle contains mobs around the level of 14 pre-hardmode and around level 64 during hardmode.
Underground jungle contains mobs around level 36 pre-hardmode.
while hardmode contains mobs around level 67.
Going later in the game you will enter the Lihzahrd Temple within the underground jungle which contains mobs around level 77.
The hallowed biome contains mobs around level 57 and only appears in hardmode and also contains and underground component.
Underground hallow contains mobs around level 60
The ocean biome contains mobs around level 28 pre-hardmode and level 55 in hardmode.
The underworld contains mobs around level 42 pre-hardmode and around 65 during hardmode.
The dungeon contains mobs around level 29 pre-hardmode. after plantera has been defeated in hardmode the dungeon contains mobs at level 70.
Upon entering a meteor crash site a prompt will tell you that the meteors are level 25.
As shown above there can be quite high level disparities. if you are skilled enough you can enter a crimson/ corruption and level up quickly to 20 if starting a new character.
It is recommended to be at least level 50 before entering hardmode as most of the mobs in hardmode are above level 55 and will hit very hard if your vitality and armour aren't high enough.
Different events have different levels.
Goblin army pre-harmode is around level 30. in hardmode the summoner is level 64.
the Pirates are around level 70
Solar eclipse is 75+
pumpkin and frost moon is level 80 +
Martian madness is level 85+
And finally the celestial event contains mobs level 90+
In expert or critical mode many of the mobs from pre-hardmode jump to level 60-70 and becoming once again threats to early hardmode players.
Killing enemies significantly lower levelled than your character will gain not much experience to the point where you only gain 1 for each kill.