What is lowest allowed entry age in Germany to participate in pc e-sports?

It really depends on the game the competition is for and who organises it. The German government, as part of the formation talks, did agree to recognise E-sports as a sport, but it is not yet clear what that exactly entails for stuff like minimum age to compete etc.

However, Germany has strict age gating rules for many games. For example, you can't play PUBG under 18, Counter Strike under 16 or League of Legends under 12. Most organisers won't allow a player who is below the legal minimum age to compete or even to spectate in order to adhere to these laws (which has lead to situations where foreign teams with different local age laws couldn't bring one of their players because they were in between the 2 ages).