I'd like to know too. I will be getting a female soon and plan to breed her with my first gecko, Bubu, who will be 9 years old when she is ready for breeding in a couple years.
From "The Herpetoculture of Leopard Geckos" book, females go into menopause in the 5th-6th year of egg laying. If can be prolonged if they are constantly laying eggs season after season.
i have heard leopard geckos breeding as old as 15 years of age. also chewbecca from here had some eight year old females lay for her the first time this year. i know a lot of geckos that breed past 5 and 6 years. i wouldnt be surprised if a 15 year old male could still get it going but it might cause a lot of undue stress on him