We've had two put down over the past 18 months. They were 14 & 15 years. One my wife adopted when she first moved here and the other was a wedding present and wonderful golden retriever. They had been part of our family since before our son was born so it was hard for everyone; but... There comes a time when it is the right thing to do. At 14 years of age and with a tumor, keeping it alive is not doing it a favor. Do the right thing, try to ensure it has a few good days and put / have it put down painlessly or at least quickly.
