There is no fixed upper age limit, it depends upon the mental kinkiness, activeness and attractivness of the individual. We have attended Fetish events where there have been active participants in their 70's, and heard of events where there are very active actors even older.
The lower age limit is fixed by laws in different countries or states.
The generally accepted minimum age for a guest at a Fetish club or party is usually no younger than 18, except where the age of consent is higher. If the age of consent is lower than 18, most clubs and parties still apply a minimum age limit of 18.
Age does not necessarily erode our kinkiness, and fresh young blood can often bring fresh new ideas into the scene.