You like to use the phrase "to end the age" and if this phrase means as what it stands for then, there is no further time.
The end of the age, is when Jesus returns to the earth physically and visual and which initiates the millennial kingdom. It is also the end of the time of the Gentiles.
After the millennial kingdom, then comes the great white throne judgment. It is there that this whole process will be completed. For following that comes the eternal state, which is what Revelation chapters 21 & 22 are about.
I might add also that according to your doctrinal belief the true believers leave this present day earth and then return again ?
Once believers are in heaven, we will be judged at the Bema seat of Christ, not for sins, which have already been forgiven, but to receive rewards, as well as suffering loss of rewards. Believers will also be in their immortal and glorified bodies which they will have received when the resurrection takes place. While up there and while God's wrath is being poured out here on the earth, we will attend the wedding of the Lamb (Rev.19:6-8), receiving our fine linen, white and clean which represents our righteous acts. At the end of the seven years, the Lord will descend from heaven and the bride/church will be following behind Him riding on white horses and wearing our fine linen that we will have received at the wedding of the Lamb.
At that time we will rule with Christ during His millennial reign on this earth.
At the end of the thousand years will be the great white throne judgment, Satan and his angels will be thrown into the lake of fire, those whose names are not found written in the book of life will also be thrown into the lake of fire. Those who will have taken part in the first resurrection will go on to rule with Christ forever and ever.
If not then who is the kingdom and to what purpose is his kingdom if there are no true believers unless somehow people become saved after the end of the age which would be quite contrary to scripture.
The purpose of the millennial kingdom, is because it has been prophesied about and Jesus was promised to sit on king David's throne:
"He (Jesus) will reign on the throne of David and over his kingdom, to establish and sustain it with justice and righteousness
from that time and forevermore." - Isa.9:7
"Behold, you will conceive and give birth to a son, and you shall give Him the name Jesus. He will be great and will be called the Son of the Most High. The Lord God will give Him the throne of His father David, and He will reign over the house of Jacob forever. His kingdom will never end!” Luke 1:32
"After this I will return and rebuild David’s fallen tent. Its ruins I will rebuild, and I will restore it, so that the remnant of men may seek the Lord, and all the Gentiles who bear My name, says the Lord who does these things" - Amos 9:11, Acts 15:16-18
It is the glory of the Lord and His kingdom on earth
* End of the age = When Jesus returns to the earth to establish His millennial kingdom
* Millennial kingdom = Jesus rules on this earth from the throne of David
* Great white throne judgment = Final judgment, the end of this earth and all of its prophesies, which is followed by the eternal state