When a level is created, he/she spawns in the creator's computer (represented by a house far from the city).

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The level spawns in at a random age from 1 to 18, most commonly spawning in at 6 - 8 years old. Usually, how the creators guess the age

The main 1.0 levels are sort of exempt from this rule, as they spawned in with the servers simultaneously. (Stereo Madness was exactly 0 years old, a newborn, on the date of release)

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The levels have collectively decided that their "birthday" should be celebrated on the day that they were brought to the city (the servers).

Hence, levels which are uploaded on the same day will have the same "birthday".
(Like Geo and Deadlocked, or Blast and ToE2)

This DOES NOT MEAN that they are twins. Just that they were uploaded on the same date. Online levels count.

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Don't even try to use their upload date to find their birth date. Not only do they not have a birth date, it is next to impossible to find out their real birthday. (Except for Stereo Madness, whose birth date is exactly on GD's release date.)