Think.

Elves.

They live forever.

Think for a while.

Forever.

What do you think it means?

It's simply just two words made into one.

"For" and "ever"

Ok, lets see what these simple words actually mean.

The word "for," for example, can mean many, many different things, just think of all the times you use this word on a daily basis.

Ok, so we looked it up in the dictionary, and there are a good 10 different definitions for the word "for." But what type of the word "for" are we looking for?

For-- conjunction for the reason that: because.

Now, lets see what the word "ever" means:

Ever-- adverb 1) always 2) at any time 3) in any way.

So basically what it means is this : 'For the reason that elves are always.'

All right, so back to the question: what does the actual word "forever" mean?

Forever-- adverb 1) for a limitless time 2) at all times: continually. noun a seemingly interminable time: excessively long.

So, in simpler words, (and back to the main point) elves live forever, they are always there, for a limitless time. There is no limit to how long they can live. They are immortal (which is actually the same thing).

Let's define "immortal."

Immortal-- adjective 1) exempt from death 2) exempt from oblivion 3) connected with or relating to immortality.

Ok... so what does "immortality" mean?

Immortality-- noun a) the quality or state of being immortal b) unending existence.

So they, in theory, never die. (Unless you kill them with a sword, but even then, they go to the Halls of Mandos, and from there they can return to Middle Earth). But they can never get sick, and thus, can't tragically die from sickness. (like in several fan fics about elves).

Ok... so... elves live forever, they never die.

Never die.

Interesting...

If they live forever, then how can elves have heirs?

Why would they need them?

Ok, now, think about this:

If you are an elf, and king of an elven country. You have a son. When would your son become king?

Answer:

Probably never, unless you decide not to be king any longer, but keep on living in the palace, or you decide to go across the sea.

Besides, why would you want to give up your crown to your son, when you can remain king of your country for as long as you want.

So...

Why do people write about Legolas being the heir of Mirkwood?

Because in the books of Tolkien, he states that Legolas is the son of the king, Thranduil.

But, do we know that Legolas is the eldest son?

No.

Why would the king send his eldest son (and heir) to Rivendell, to tell Elrond that they lost Gollum?

He wouldn't.

He would most likely send one of his other sons because they (though still carrying the family name) won't eventually be the king of Mirkwood.

Wait...

Then why would hundreds of fanfiction writers write about how Legolas is the heir to the throne of Mirkwood?

And why would it make a difference anyway?

He will never be king, because his father an elf.

And elves....

Live forever.