We've generally lived by the assertion that men with gray hair should keep it short. When gray hair gets long and too out of control it runs the risk of becoming crazy, nutty professor hair.
George Clooney's length in the "The Descendants" is about as long as you want to take it. But also consider this, he's George Clooney. Unless you're George's general level of hotness, you probably won't be able to pull off a style any longer than this!
Richard Gere can pull off a long haired look with gray hair. But consider again, he's Richard Gere. So unless you are Richard's general level of hotness, you probably can't pull off his length of grayness.
And long white pony tails are a no-no! Willie Nelson is already working that look. Leave it to Willie. Sorry. If you insist on wearing it long, don't hide your gray glory, share that mane with the world!