Heart Murmur diagnosis

I have had two elderly dogs with heart murmers - one a GSD and one a JR. Neither had a severe murmer and it was just a watching brief - the vet said to be particularly vigilant about them coughing when they were at rest, I seem to remember. Both dogs lived to a good age (GSD 13 and a half) and JR (16 and a half). For neither of them were their hearts the reason that we had to make the decision to have them PTS.
Old dogs are your most wonderful and faithful friends and it is the worst part of dog ownership watching your loved, loyal companion becoming old and infirm.
Enjoy every day - store up memories and love your old one - helping them to keep cool in the heat and warm in the cold is one way that you can make life more comfortable for them. Ensuring that they maintain a healthy weight is important too - some old dogs pile on the pounds and others get skinny - like people.
In my opinion your dog wants most to be with you - as much as possible and that is what makes them the most happy. So as long as that is possible you enjoy your dog

Our cockapoo Simon was diagnosed with a stage 3 murmur when he was 6 or 7. Back then we just kept up vet visits and monitoring. As time went by the murmur progressed until it was loud enough to hear if you just put your head down at his level. The murmur turned into severe arythemia in both chambers of his heart and his blood flow could not keep up with his body's needs. Simon passed away in May at 13.5 years old. In hindsight, I think I would have had the baseline tests done, electrocardiogram, etc., and perhaps he could have made it longer. on different meds. He took 3 at the end - Vetmedin, enilapril, and lasix. The good news (if it can be considered good) is that this condition was not painful for him. It did, however, restrict his activity some, as we needed to carry him up the stairs when he had to go out. I'd say just watch your little one closely, and if you can, bring him to a specialist.