I'll just jump right into it. Suppose a player is the most experienced player in a team and in the league. With a season's worth of matches played, is the experience he/she gains as the seasons go by static? Or does it increasingly get smaller the higher in experience they go?
Factors that affect experience gain of players from a match directly depends on:
1. Minutes played by the player - more minutes, more experience gains
2. Average experience of all 22 players on the pitch (your own 11 players in case of custom matches/friendlies) - more average experience, more experience gains
3. The player's current experience - more experience, less gains
So to answer your question:
As stated in point #3, as a player gains more and more experience, his gains from matches will start to slow down. But he will still keep gaining something until his experience is at maximum possible value.
And what is maximum value ?