To me, lymphocystis looks more like cauliflower than those round growths (?) or patchy white areas...but...
I hate to say, but after 13 or 15 years, your fish is quite old. No fish or mammal dies of "old age" alone. It's always a failure of internal organs, immune system, cancer or more prone to diseases of any type, etc. You've given it a long life so far. The only thing I can say is do lots of water changes often. If it has food preferences, give them often too.
I'm sorry I don't have better advice.