Averagefellar, I love your name. I’ll try to answer your questions:
1. Five-point Calvinists say that non-Christians are unable to repent of their sin and place their faith in Christ because they are spiritually dead. Remember Ephesians 2:1: “And you were dead in your trespasses and sins.” How can a spiritually dead person do anything that is spiritually significant? Total depravity implies total inability. Jesus made it clear, however, that somehow a dead person could hear the gospel and believe in God (John 5:24-25):
“Truly, truly, I say to you, he who hears My word, and believes Him who sent Me, has eternal life, and does not come into judgement, but has passed out of death into life. Truly, truly, I say to you, an hour is coming and now is, when the dead shall hear the voice of the Son of God; and those who hear shall live.”
Some people argue that the dead people mentioned in verse 25 refer to physically dead rather than spiritually dead people, but the proximity to verse 24 and the clear reference to physically dead people in verse 28 seem to rule out that interpretation.
A common analogy used to represent depravity is drunkenness. A drunk person is unable to drive safely, and a depraved non-Christian is unable to surrender his life to Christ in repentance and faith. Again, total depravity implies total inability. A person inclined to sin cannot truly repent of that sin unless his depravity is somehow counteracted. Is a person sober before he gets drunk and drives? Yes, and that is why society can hold him responsible for driving drunk. If an evil gang forced a person to drink and drive, then society would not hold the drunken man responsible for an accident.
God holds humans responsible for their depravity because they were perfectly represented by Adam when he committed his first sin. If Adam did not have true free will, then he could not be held responsible for his sin, and his descendants also could not be held responsible for the sins they commit while they are in a totally depraved condition. Humans are also held responsible for the ultimate, final choice they make to accept or reject Jesus as their Lord when they are under the special, illuminating conviction of the Holy Spirit. As Adam was held responsible for his free will choice, so his descendants are, also.
The non-Christian’s conscience may be functioning from birth, but every part of him is affected by depravity, including the conscience. The Holy Spirit must activate and calibrate it during the time of special conviction. At that time the effects of depravity are temporarily counteracted so that the non-Christian can self-generate a bias from an unbiased position (equipoise) and surrender to Christ in faith and repentance.
2. The age is not specified chronologically. Some people who are severely mentally handicapped and infants dying in infancy never reach that age.