Hello, where can ifind informations about system decay?

The decay system applies to 2400 and + players as far as I know.

RM 1v1: http://www.voobly.com/ladder/view/Age-of-Empires-II-The-Conquerors/RM-1v1
RM TG: http://www.voobly.com/ladder/view/Age-of-Empires-II-The-Conquerors/RM-Team-Games

On the right box of “Information”, you have your answer of how it works.

However, the decay system applies to 2k+ players