Does proliferate work on experience counters?

Proliferate allows you to choose any number of permanents and/or players that already have at least one counter on them, then put one additional counter of each kind already there onto each chosen permanent or player (CR 701.27). Experience counters are counters that exist on players, so they qualify.

The key requirement is that the player must already have at least one experience counter. Proliferate does not grant counters to players or permanents that have none — it only adds to existing ones (CR 701.27b). If a player has zero experience counters, proliferate cannot give them one.

Experience counters are explicitly defined as a type of counter that can exist on a player (CR 122.3). There is no rule restricting proliferate from interacting with experience counters specifically; any counter type on a player is eligible.

Example: You control Meren of Clan Nel Toth and have accumulated 3 experience counters. An opponent casts Inexorable Tide, which has proliferate. When they proliferate, they may choose you as a player with a counter, adding a 4th experience counter — making Meren's ability even stronger.

Note that proliferate adds exactly one counter of each type already present per chosen player or permanent (CR 701.27b), so you cannot add more than one experience counter per proliferate trigger unless a replacement effect modifies that.