Hello myid®

The Level (Oldtimer Score)  has nothing to do with scoring in a game. 
Instead it is a system primarily based on total playtime, designed to reflect a player's long-term commitment and conduct.

Demerits can, and bans will, negatively impact the a player's level (Oldtimer-Score) by deducting points accordingly. 
If all points are lost, a player drops back to level 0. 

However, it can be restored over time simply by continuing to play and staying penalty-free.  

(When the Oldtimer Score is negative, the progress bar on a player's profile will not show until the score returns to a positive value.)