As we grow older most of us become more set in our ways, and we aren't as open to change as we once were. But coming to Christ involves a radical change, for God calls us to turn "from darkness to light, and from the power of Satan to God" (Acts 26:18).
But there is a deeper spiritual reality at work here also: The more we turn away from God, the less sensitive we become toward Him. Just as our hands will grow hard and calloused by constantly rubbing against a tool, so our hearts grow hard and calloused when we repeatedly resist God. We become like those of whom God spoke in the Old Testament: "They did not listen to me or pay attention. They were stiff-necked" (Jeremiah 7:26).
Yes, most people do come to Christ when they are younger and more open to the gospel. That is why we need to pray for our youth and for those who work with them in our churches. We also need to pray for our families and ask God to help us point our children (and our grandchildren) to Christ.
Pray also for those around you who are older but seem indifferent to God. Their time grows short, and they need Christ more than ever. God loves them, and by His Spirit He can break through their spiritual hardness.