so I can work out,eat a low fat high fibre diet and abstain from alcohol -and expect my heart to be the same age and expect to live as long as a beer swilling,fag smoking couch potato
I don't think that's what it's measuring. It's telling you your life expectancy right now on the basis of the way you've lived so far, as measured by things like cholesterol. If you're 50 and have lived an exemplary lifestyle your heart age - now - should be lower than if you're 50 and have pigged out for years. The way you live in future will affect things further.