I am curious to know what you say the "end of the age" refers to? Does it refer to the end of the world or to a specific age? I ask this in relation to what you said about it in Matthew 28.
I think some people (Full Preterists/Hyper Preterists) believe the "end of the age" refers to the destruction of Jerusalem.
Matthew 13 talks about the harvest being the end of the age, but it also mentions the harvest being of God's kingdom, so if the harvest is the Judgment of God, then what about the non-Christians who aren't part of the kingdom, in the context of this specific passage?
"Age" is from the Greek word aion, it has a variety of meanings, but for this context, it refers to a large period of time or an epoch. "The end of the age" is a phrase that Matthew uses.
"Then Jesus sent the multitudes away, and went into the house. His disciples came to him, saying, "Explain to us the parable of the darnel weeds of the field." He answered them, "He who sows the good seed is the Son of Man, the field is the world; and the good seed, these are the children of the Kingdom; and the darnel weeds are the children of the evil one. The enemy who sowed them is the devil. The harvest is the end of the age, and the reapers are angels. As therefore the darnel weeds are gathered up and burned with fire; so will it be at the end of this age. The Son of Man will send out his angels, and they will gather out of his Kingdom all things that cause stumbling, and those who do iniquity, and will cast them into the furnace of fire. There will be weeping and the gnashing of teeth. Then the righteous will shine forth like the sun in the Kingdom of their Father. He who has ears to hear, let him hear"" (Matthew 13:36-43).
You mixed terms, so it resulted in some confusion. The field is the world. The good seed in this parable is God's kingdom. The kingdom exists for the time in the world, but Satan has also seeded the world with his own followers and during this age, they intermix. The harvest is the end of the age and the reapers are the angels. It is at the end of the age that the righteous are separated from the wicked. The wicked will be destroyed and the righteous will shine forth.
The point is that though Jesus established the church in the world, Satan is still working against God. If the wicked were destroyed now, the righteous would be harmed as well. So God will wait until the end of the age to separate the good from the bad.
In this context, "end of the age" obviously refers to Christ's second coming. "Since it is a righteous thing with God to repay with tribulation those who trouble you, and to give you who are troubled rest with us when the Lord Jesus is revealed from heaven with His mighty angels, in flaming fire taking vengeance on those who do not know God, and on those who do not obey the gospel of our Lord Jesus Christ" (II Thessalonians 1:6-9). It cannot refer to the destruction of Jerusalem since there was no separation of the righteous from the wicked, nor have the wicked been destroyed.
"Again, the Kingdom of Heaven is like a dragnet, that was cast into the sea, and gathered some fish of every kind, which, when it was filled, they drew up on the beach. They sat down, and gathered the good into containers, but the bad they threw away. So will it be in the end of the world. The angels will come forth, and separate the wicked from among the righteous, and will cast them into the furnace of fire. There will be the weeping and the gnashing of teeth" (Matthew 13:47-50).
Here the illustration shifts a bit. The dragnet is the kingdom and it pulls in all sorts, both good and bad. At the end of the age (world in this translation, but it is the same Greek word), the angels will separate the good from the bad. The righteous will be kept, but the wicked will be destroyed.
The point here is that the church by its design draws in both good and bad people. It is designed so to gain the greatest number of good people. At the end, the wicked will be separated out. Again this "end of the age" cannot refer to the destruction of Jerusalem. The church still exists. It still has good and bad people in it. And, the wicked have not yet been destroyed. But it does match the description of the end of the world and final judgment.
"As he sat on the Mount of Olives, the disciples came to him privately, saying, "Tell us, when will these things be? What is the sign of your coming, and of the end of the age?"" (Matthew 24:3).
When Jesus told the disciples that Jerusalem would be destroyed, they wanted to know when this would happen. In their minds such as destroy would mean the end of the age, but as we see going through the chapter that Jesus explains that the destruction of Jerusalem and the end of the age are two separate events. See The Fall of Jerusalem and The Second Coming of the Lord for more details.
"Jesus came to them and spoke to them, saying, "All authority has been given to me in heaven and on earth. Therefore go, and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, teaching them to observe all things that I commanded you. Behold, I am with you always, even to the end of the age." Amen" (Matthew 28:18-20).
If Jesus promised to be with the disciples always, even to the destruction of Jerusalem, the two phrases do not match. Forty years is not "always." Nor would there be great comfort in knowing that Jesus would be with them for only the next forty years.
Thus every time "end of the age" is used in Matthew it points to the same event: the end of the last age, the Christian age, judgment and the end of the world.