The traditional gift for a 50th Wedding Anniversary is gold. Gold is a metal that is durable so I suppose that the symbolism is that of something that has lasted fifty years, is very valuable and has survived the ups and downs of life and is still standing and hopefully in good condition. There are not many things that increase in value over the years but a good marriage does, as does gold.
Life by its very nature is moving towards days of decline. Over time we lose strength, we are not quite as sharp as we used to be and one day life will ebb away and the question is, what then? One of the reasons I am a Christian is because the Lord Jesus Christ has given me eternal life. The biblical definition of eternal life is twofold. 1. It is the spiritual life that a person gets the moment their sins are forgiven and they have made their peace with God plus 2. It is the guaranteed prospect of existing forever in heaven (with a new body at the resurrection) in a place where death, sickness, sorrow and the effects of evil can never touch the people who are there.