[user104658]

Originally Posted by Alf

Flat racing is more dangerous than jump racing, because they're running at a much faster speed. That's why on average flat track horses will retire before they reach the age of 5 years old, many before that age. Grand national horses range from about 7 year old to 15 year old.

This has nothing to do with how safe it is; it's because pure speed is important on the flat whereas experience comes into play with the jumps. Younger horses are faster on the flat, thus they get retired from flat racing earlier when they become too slow to compete. Jumps are (quite obviously) much more dangerous for both horse and rider.