In this video pastor John Piper answers a question: How old should elders be?
The Bible doesn't give a specific number. Perhaps Timothy in the Bible was an elder when the Apostle Paul wrote to him, "Don't let anyone look down on you because you are young, but set an example for the believers in speech, in life, in love, in faith and in purity." - 1 Timothy 4:12
So the assignment of elder should be based on spiritual maturity and meeting the qualifications found in the Bible.