It's a great question. Esselstyn seems to get very close to 100% remission of heart disease no matter your age or history, but he can't reverse nerve or muscle damage from a previous heart attack.
I've worked with 12 people who had serious heart disease over the last 10 or so years. Some had previous heart attacks, two had previous bypass operations, my father-in-law had surgery on a valve in his 80s. His cardiologist had him on the paleo diet and he had threatening symptoms.
6 of them read the books, a couple of them called Esselstyn, my father-in-law watched What the Health and Forks Over Knives, and they all gave up animal and refined foods. Far as I can tell, they're all doing really well. My fil is 88 and is out walking, roofing his house, etc. No symptoms anymore.
Of the other 6, most of them heeded the advice of their cardiologists and got stents or bypass operations. Two of them have died of strokes. One is doing okay in his 80s but I worry because he's got a big belly and his bypass was almost 10 years ago. The others just don't look or feel well, but they're alive and eating their favorite foods which they say is very important to them.
It looks like the same mechanism that causes heart disease is responsible for most strokes and Alzheimer's. Once Alzheimer's sets in, I'm not aware that it can be reversed.