Apostle Paul believed that even among the young people in the Church a pure heart was not something impossible. He certainly was aware of such saints. He said to the young man Timothy:
22 Flee also youthful lusts: but follow righteousness, faith, charity, peace, with them
that call on the Lord out of a pure heart.
2 Tim. 2:22
He did not think Timothy would be alone in living a life of purity of heart.