You could say: My elderly good friend...or my elderly friend...
By saying this, there is no confusion, because you are only saying that your friend is old of age but not saying that you have known him for a while like you would if you said "My old friend". Or if you really want to be poetic :) you could say: " My dear friend, who is old of age but not of soul, ....."
Hope I helped, if you think I can help more, feel free to look me up on Italki. Cheers!