How old was Gimli when...

[Legolas]

In that line of thinking, nor does Tolkien writing that the Fellowship was made of four hobbits, two men, a wizard, a dwarf, and an elf mean that you have to accept it.

Depends on if you go by what Tolkien wrote, or what you wish his story to be like. I am 'inclined' to the first, since he wrote it and I didn't.

As adequately explained by Sharku,

I see no point whatsoever, in any form of literary research, to question a 'fact' Tolkien gave us, within or without the fiction, since anything relating to the Legendarium is necessarily part of the fiction. What would be the point of denying the authority of a quote such as the one from the Letters, explaining that Sauron was of human form? One might get a different idea of Sauron when reading the books, and that in itself is interesting, but the fact is part of the whole. If a reader refuses to acknowledge certain parts of a work of fiction (or actual fact, for that matter), there's nothing I can do about it, but that person's position in a discussion forum is difficult to say the least. Conversely, I doubt someone with a clear idea of his own which one is unwilling to give up would ask such a question anyway.

So...Tolkien said they made it and died peacefully sometime later. Whether you believe him is up to you, but there's little significance in not believing him outside of your own head.