OLD IS GOLD
As the years go by we grow in age. If you let your mind to grow old you would vegetate. You become good for nothing.
An active mind makes people achieve quite a lot , much beyond your own imagination, to the surprise of everyone including yourself. People learn from experiences. The old (as others state) have a wealth of knowledge. If you don't share that it goes waste, into the dustbin of history.
When you pass on what you have learned to others you are doing a great service to humanity. If you don't do that it's a great disservice. You are not fulfilling the purpose for which you have been created. The creator is supreme, whether you like it or not, and fulfilling the mission and vision of the creator is paramount.