What does Mark 10:20 mean?

"Youth" is from the Greek root word neotes. It just means "youth" and doesn't refer to a specific age. Judaism teaches that if a young child sins, the punishment will be taken on his or her parents. In Jewish tradition, a boy became responsible for his moral decisions at the age of twelve. So, the young man is probably saying that he has kept all these commandments since that point. He has only kept the commandments, however, in a literal way. Jesus pointed out in the Sermon on the Mount (Matthew 5:21–48) and just recently (Mark 10:1–12) that there is a difference between obeying the law and fulfilling it. Those who willingly fulfill the spirit of the law do so because they love and trust the law-giver more than any worldly benefit they would receive otherwise. This is something the man cannot do (Mark 10:22).

The apostle Paul makes a similar claim to virtue. He says that if righteousness is measured by how well one kept the law, he is blameless (Philippians 3:6). But he also explains that it is not his own righteousness, defined by the law, that earns him resurrection from the dead, but the righteousness that comes from God and is expressed as faith in Christ (Philippians 3:9–11).

The rich young man's assertion reveals a great irony. From his youth—the age of accountability—he has done everything he can think of to earn eternal life. This is quite a remarkable feat. But Jesus has explained that we must go back to when we could not be accountable for our sins. Shortly before, Jesus had said, "Truly, I say to you, whoever does not receive the kingdom of God like a child shall not enter it" (Mark 10:15). Salvation has always come in response to our understanding that we have no standing with God and we must rely on Him (Hebrews 11). Obedience to the law has always come in response to our love and respect for God (Exodus 20:6; Deuteronomy 11:1; Nehemiah 1:5; John 14:15). The law doesn't give us an opportunity to earn salvation, it proves that we can't.