Get a second opinion.

Just because his knees are shot... doesn't mean yours will be. There are so many factors there.

Where did he run (hard or soft surfaces)? What kind of miles did he put in each week (did he train for marathons? Or short distances?)? Did he have good form (poor form = hurt knees!)? How fast did he run (was a speed demon?) Etc. etc.

Look, he's not God. He doesn't have all the answers. Don't let what he said discourage you.

Have you started running? Do your knees hurt? Have you tried running slow?

Many people run into old age, and do OK. Go get a second opinion.