I have a 13 year old and her record is as follows:
Age 3 - 9 eggs (no second clutch)
Age 4 - 25 eggs (16 first clutch, 9 second clutch)
Age 5 - 31 eggs (20 first clutch, 11 second clutch)
Age 6 - 33 eggs (22 first clutch, 11 second clutch)
Age 7 - 37 eggs (26 first clutch, 11 second clutch)
Age 8 - 32 eggs (19 first clutch, 13 second clutch)
Age 9 - 44 eggs (24 first clutch, 20 second clutch)
Age 10 - 39 eggs (26 first clutch, 13 second clutch)
Age 11 - 36 eggs (21 first clutch, 15 second clutch)
Age 12 - 37 eggs (24 first clutch, 13 second clutch)
Most of the first clutches have been fertile all through her life. However, over the last three years the proportion of slugs in the second clutches has gradually increased.
To my mind, she was at her peak between 7-10 years.