Here comes some old talk from an old former pastor . . . From a human relations standpoint, anytime there is a 20 year gap or more between the age of the former pastor and the new pastor, the church is in for a bumpy ride. That does not mean that it is not of God or that he is not called to the church. But, human nature being what it is, more than a 20 year gap and you are in for some interesting times. I say this as one who followed two older pastors in my ministries.
We all tend to affiliate with (i.e., socialize, make friends with) people around our own age. When a much younger pastor comes onto the scene (even without a merger) the power brokers who were "buds" with the former pastor are likely to feel frozen out of the inner circle by the new kid on the block. Unless they are VERY mature in their faith, this will give way to fault finding and criticism driven by emotions that they may not even fully realize or acknowledge. You would be surprised how many things a pastor can be found doing "wrong" when people are strongly motivated to find fault. Add to this the very different corporate cultures of the two congregations and wowie zowie. This is a likely to be a VERY exciting Disneyland ride.
Pray for both congregations as they continue to seek God's will. There is a very fine line between energizing synergy and enervating sin-ergy. The new life to an older congregation could be a tonic. However, that does not mean that it will be easy.