Gandalf is vastly older than Legolas; Gandalf is an ancient spirit (a Maia) who existed before the world, while Legolas is an elf who, though hundreds of years old, is relatively young, existing only for a fraction of Gandalf's lifespan, making Gandalf the far more ancient being by millennia.
His true age isn't precisely stated, but he was around long before time was measured in Middle-earth, so thinking in tens of thousands of years isn't far off. It's Tolkien's way of emphasizing Gandalf's timelessness and deep knowledge.
The Wizard clad in brown was Radagast and the one clad in grey was Gandalf, seemingly the oldest and the least of the Order. The other two who travelled to the East and South were sometimes said to clad themselves in robes of sea-blue and were known as the Blue Wizards.
It's not the years; it's the mileage. Gandalf isn't a Maia in the same way Sauron is. He was sent to middle earth as an old man, so his body is much more real than Sauron's “raiment”. He can't use his power to the same extent, he can't change his forms at will.
Sauron, Gandalf and Saruman are approximately the same age. Each appearing before Eru's first music. Galadriel by comparison was born in 1362 YOT in Valinor.
In Valinor, Gandalf, a Maia, was named Olórin, one of the people of the Vala Manwë, and the wisest of the Maiar. He was closely associated with two other Valar: Irmo, in whose gardens he lived, and Nienna, the patron of mercy, who gave him tutelage.
Arwen married Aragorn in the year 3019 of the Third Age, when he was 88 years old and she was 2,778 years old. The last year of the Third Age was 3021. Aragorn ruled until the year 120 of the Fourth Age, when he was 210 years old.
It's possible that Gollum's prolonged search for the ring allowed it to maintain a significant power over him, suspending his aging process. In contrast, Bilbo's exposure to the ring's influence waned after he willingly (mostly) relinquished it, allowing his aging to resume more noticeably.
Power was never going to succeed in defeating evil. Gandalf was at exactly 12.35% of his full power as Gandalf the Grey, this went up of 18.237% as Gandalf the White. The Maia varied greatly, as did the Valar. Even at his fullest, Olorin/Gandalf was not the equal of Mairon/Sauron, not even close.
The Ent who figures most prominently in the book is Treebeard, who is called the oldest creature in Middle-earth.
Frodo comes of age as Bilbo leaves the Shire. Frodo inherits Bag End and Bilbo's ring. Gandalf, uncertain about the origin of the ring, warns Frodo to avoid using it and to keep it secret. Frodo keeps it hidden for the next 17 years, and it gives him the same longevity and youthful appearance it had given Bilbo.
He dies at the age of 210, after 122 years as king. The graves of Merry and Pippin (who had died in Gondor 58 years earlier) are set beside his. He is succeeded on the throne by his son, Eldarion. Arwen, heartbroken by the loss of her husband, dies shortly afterwards in Lothlórien.
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In the movies, there's a scene where Eowyn specifically talks about how Aragorn looks SO much younger than his actual age of 87. Peter Jackson mentions that the previous actor for Aragorn was "a little too young," so they switched to Viggo Mortenson (who was in his early 40s).
In Peter Jackson's "The Lord of the Rings: The Two Towers," the wizard says, "300 lives of Men I've walked this earth, and now I have no time." While this gives the impression of longevity, measuring by lifespans is vague and inconsistent; many of Aragorn's Númenórean ancestors lived for multiple average lifespans.
Why was Sauron afraid of Aragorn potentially possessing the Ring? Gandalf explains it in the Return of the King (during the council after the battle of the Pelennor fields) that Sauron would never think somebody would want to destroy the One Ring.
His task was to do the bidding of his masters and so when he proved unfaithful in doing that task his masters stripped him of his robes and gave them to one who would do their bidding. Gandalf is now the White, Saruman as he should have been.
Gandalf is not permitted to use his full power during the 3rd Age (some exception apply). He has been sent to guide humans to the 4th age, the age of mortals. He can perform minor magic as needed, but he cannot "bust out."
Tolkien's description of Gollum conforms to a Catholic – and Thomistic – account of envy, which is a “sadness of the soul”; and it is Gollum's unbearable sadness and his unquenchable desire for the one Ring that marks his character.
In the books Bilbo hadn't even come close to catching up with his biological age until after the Ring was destroyed. (And since Gollum was destroyed with the Ring, there wasn't any chance for his age to catch up with him.)
Instead, we have to consider that Sauron, the original owner, is not human, but has a quasi-divine status. Therefore ownership of the Ring is subject to Divine Law. With this precedent in mind, many of the ambiguities clear up nicely. After numerous transfers by violence and trickery, Frodo is the true owner.
A recent post got me thinking about Aragorn and Arwen's children. He has one son, Eldarion, who takes over the United Kingdom once he passes on. And they have two unnamed daughters.
Éomer: 28 years. Éowyn: 24 years. Diamond of Long Cleeve: 24 years. Lothíriel: 20 years.