I dont use the word "kit". It sounds wrong. Ok for first aid kit and tool kit and pack up your troubles in your old kit bag, but, I prefer to call my windsurfing stuff, "gear"...(or stuff it would seem)
..my oldest bit of gear would be my Maui Project wave board.
.
1992 Mistral Mast base and extension rubbers replaced, 1988 Shane O'Callahan 9'2" Slalom from S.A badged under Island Energy Tasmania (sentimental)
I bought my rig in 1987 (second hand) had it ever since, although I did not use it between 1990 and 1997 then again not between 1999 and 2006.
But since 2006 I have used it regularly, although due to wear and tear I have had the replace various bit over time i am not sure exactly but think I have replaced the sail 3 or 4 times, the mast twice and the boom 4 times, while the extension and UJ have only been replaced once..Oh sorry maybe twice. Thats the great thing about quality kit, if you maintain it properly it will last forever.