HCMW, I remember De Pluvinel (just!). He won the Royal Artillery Gold Cup as a 17 year-old at Sandown in 1990.
You might also remember the gallant grey Eastern Destiny who won a hunter chase as a 17 year-old at Nottingham in 1995. Sadly he collapsed and died after the race, though hopefully he went out doing something he enjoyed. He had plenty of other successes in his teens, and still appeared to have a zest for racing.
I’m not aware of any successes at such an extreme age since, though there might have been the odd one or two in points.