My oldest mare is 30 years old (31 in April) and has a heart murmur. In the last few years it's gotten worse, and it's now Grade 5, whatever that means. In her 20's, I drove her many miles on our back roads (pulling a 2 wheel cart) without a problem. She loves her work and is still the Queen for pony rides and does so happily!
You should ask your vet some more questions to learn whether it's a serious murmur, or incidental, and whether there are restrictions on his activity. I think a lot of horses (and people, too) have mild heart murmurs that don't cause them any problems.
My vet said my mare would live long enough to give pony rides to my grandchildren (I have none yet!). We baby her because she has arthritis in her shoulders and knees, so she isn't used for hard work anymore.
Last edited by Ladytrails; 01-07-2012 at 10:19 PM.