This diagram shows how loud certain frequencies must be in order for people of different ages to hear them. The needed intensity is called the "hearing level."

A graph with age in years on the x axis from 20 to 70 and hearing level in decibels from 30 to 0. There are 5 graphs that are arcs that all start from point (25, 0) and end at points (70, 10), (70, 14), (70, 15), (70, 20), (70, 25) and (67, 35) and are labeled 500 Hertz, 1000 Hertz, 2000 Hertz, 3000 Hertz, 4000 Hertz and 6000 Hertz respectively.
According to the graph, what is the maximum age at which a 4000 Hz sound wave with an intensity of 20 dB would be heard?

55
60
65
70