Thank you for the question of age limits to enter heaven or hell, “What is the age limit to enter heaven or hell in different scriptures?”
The answer which I am suggesting is my own observation of the Scriptures. It does not represent other Christians or churches.
As one reads the Scriptures, there is no one who deserves to enter heaven and everyone desires to enter hell or the Lake of Fire. It is not a matter of age of the person but the condition of the person.
A. The condition of every person who is born into this world has a sin nature and will sin against God or others.
Romans 3:23 says this, “For all have sinned and come short of the glory of God” (A searchable online Bible in over 150 versions and 50 languages).
Romans 3: 10 says this, “There is none righteous, no not one” (NIV).
Eph. 2:8–9 says this, “For it is by grace you have been saved, through faith—and this is not from yourselves, it is the gift of God— 9 not by works, so that no one can boast” (NIV).
The Apostle Paul argues that no human being can by his own good works earned heaven. This includes any baby in their crib or an senior on their deathbed.
b. The condition of every person who receives salvation is by faith.
John 1 says this, “11 He came to that which was his own, but his own did not receive him. 12 Yet to all who did receive him, to those who believed in his name, he gave the right to become children of God— 13 children born not of natural descent, nor of human decision or a husband’s will, but born of God” (NIV).
I John 5 says this, “9 We accept human testimony, but God’s testimony is greater because it is the testimony of God, which he has given about his Son. 10 Whoever believes in the Son of God accepts this testimony. Whoever does not believe God has made him out to be a liar, because they have not believed the testimony God has given about his Son. 11 And this is the testimony: God has given us eternal life, and this life is in his Son. 12 Whoever has the Son has life; whoever does not have the Son of God does not have life” (NIV).
Mark 10 says this, “People were bringing little children to Jesus for him to place his hands on them, but the disciples rebuked them. 14 When Jesus saw this, he was indignant. He said to them, “Let the little children come to me, and do not hinder them, for the kingdom of God belongs to such as these. 15 Truly I tell you, anyone who will not receive the kingdom of God like a little child will never enter it.” 16 And he took the children in his arms, placed his hands on them and blessed them” (NIV).
Romans 10 says this, “If you declare with your mouth, “Jesus is Lord,” and believe in your heart that God raised him from the dead, you will be saved. 10 For it is with your heart that you believe and are justified, and it is with your mouth that you profess your faith and are saved. 11 As Scripture says, “Anyone who believes in him will never be put to shame.”[e] 12 For there is no difference between Jew and Gentile—the same Lord is Lord of all and richly blesses all who call on him, 13 for, “Everyone who calls on the name of the Lord will be saved” (NIV).
The answer to the question is when a child or person is able to understand and say, “I am sorry God for my bad conduct. Please forgive me of my sins Thank you Jesus for paying/dying for my sins so that I can spend forever with you.” It is likely that a 3 or 4 years may be able to understand Jesus loves them and died for their sins.
What happens if an infant dies before he hears or understands the Gospel of salvation or if a person never hears of the saving grace of Jesus Christ. For infants, the Judge of all the earth will do what is right for infants. For people who never heard the Gospel of salvation, God will judge each person by that person’s own conscience. It will either excuse him/her or accuse him/her of one’s belief or values.
SUMMARY: It is not about the age of the person, but the condition of the person’s heart toward God and others.
For more perspectives:
https://www.quora.com/What-is-the-age-limit-to-enter-heaven-or-hell-in-different-scriptures