With the end of the Third Age began the Dominion of Men. Elves were no longer involved in Human affairs, and most Elves left for Valinor; those that remain behind "fade" and diminish.
The Fourth Age was the time period that followed the War of the Ring (immediately) and the Third Age. It was a time of peace, and was also known as the "Age of Men".
The reasons for the Elves inviting him are clear given his destruction of the One Ring and Sauron, but Frodo's reasons for accepting are less so. Ultimately, it was the injuries that the ring-bearer Frodo endured on his journey that pushed him to leave Middle-earth.
The Third Age was the Age of Arda that lasted for 3021 years, from the first defeat of Sauron by the forces of the Last Alliance and the deaths of Elendil and Gil-galad, to the departure of Gandalf, Galadriel, Elrond, Bilbo, and Frodo to the Undying Lands after Sauron's final downfall during the Great War of the Ring.
Sauron's influence also caused the Drowning of Númenor and the Changing of the World. The Age ended with the War of the Last Alliance and the defeat of Sauron.
“Fourth age” is a term used in gerontology to describe the last stage of life, which follows the “third age” of active, healthy, self-directed retirement (Lloyd, 2015).
The Fourth Age was held to have begun with the passing of the Ring-bearers over the sea from Mithlond on 29 September III 3021, though in Gondor it was reckoned as beginning on 25 March of the same year (that date being the second anniversary of the Downfall of Barad-dûr).
The Dark Tower, the Black Gate and the Towers of Teeth collapsed to ruin. Mount Doom exploded. Both Sauron and his Ringwraiths were apparently destroyed. After the ultimate defeat of Sauron, Mordor became mostly empty again as the Orcs inside it fled or were killed.
He will fight in the Last Battle against the Valar and their allies, but will ultimately be slain by Túrin Turambar, the Man he cursed. By finally defeating Morgoth, Túrin will avenge not only himself, but all members of the race of Men.
The Fourth Age began after Sauron was finally defeated, when his Ruling Ring was destroyed, and the Keepers of the Three Rings left Middle-earth for the Uttermost West.
And Tolkein himself confirms this in his own private letters, where he makes it clear that Frodo and his friend Sam (who is also eventually granted a place in Valinor) would always be mere mortals and would eventually succumb to death even if they were to remain in The Undying Lands.
Though the Undying Lands can be viewed as heaven, going there doesn't mean death. It is a paradise, yes, but it is not necessarily the end. As far as we know, those elves go on to lead beautiful immortal lives in the Undying Lands.
Where did Frodo go at the end? Frodo goes to the Undying Lands with the elves. While the mortal human, hobbit, and dwarf characters discuss death and dying (and do plenty of it) in Lord of the Rings, the immortal elves don't talk about their end in the same manner at all.
What this means is that Frodo's traumatic injuries would always come back to haunt him every year. The only way for Frodo to find peace was for him to leave Middle-earth and head to the Undying Lands, in the hopes of finally healing his injuries.
Not much is known about the Dwarves after the Third Age. After the War of the Ring, Gimli brought a part of Durin's Folk from Erebor to the Glittering Caves behind Helm's Deep and founded a colony there. Subsequently, Gimli went on many travels with his friend Legolas, and History lost track of their fate.
They were also joined by Frodo and Bilbo Baggins, Hobbits who were among the very few mortal beings to be allowed passage to the Undying Lands. Eventually, Samwise Gamgee, another Hobbit of the Shire, and the Dwarf Gimli along with his good friend Legolas, are also said to have made the journey.
Morgoth's power far outstripped that of Sauron, at least at the beginning. Therefore, it is evident that Sauron did not regret serving Morgoth. He embraced evil and darkness as his own, and he pursued his goals with passion and cunning.
Morgoth looms over The Lord of the Rings: The Rings of Power. Though the War of Wrath ended with Morgoth's defeatOpens in new tab, his touch spreads across all of Arda into the Second Age and beyond.
The most powerful of the Valar other than Morgoth were Manwë (meaning “blessed one”), who became their king, and his wife, Varda (“sublime one”) who became queen. Manwë's area of dominion is the air and the winds, and the Great Eagles are his servants and messengers.
According to Tolkien's writing, Galadriel didn't fight in the War of Wrath because she didn't believe Morgoth could be defeated without the help of the Valar.
Frodo Baggins, son of Drogo Baggins, was a hobbit of the Shire in the late Third Age.
Frodo Baggins was a hobbit of the Third Age, the most famous of all Hobbits in the histories for his leading role in the Quest of the Ring. During this epic quest, he bore the One Ring to Mount Doom and there destroyed it, giving him renown like no other Halfling throughout Middle-earth.
The Undying Lands were a realm inhabited by Ainur and Eldar. The area included the continent of Aman and the island of Tol Eressëa. The ocean Belegaer separated the Undying Lands from the western shores of Middle-earth. Only immortals and ring-bearers were allowed to live in this realm.
After the death of King Elessar, Legolas made a ship in Ithilien, and through Anduin, he left Middle-earth to go over the sea. His strong friendship with Gimli prompted him to invite Gimli to go to the Undying Lands; making him the first and only Dwarf to do so.
According to that definition, the First Age 'proper' followed the Years of the Trees and lasted only c. 600 years until the beginning of the Second Age.