noruiwen
Because elves are immortal? Legolas is at least quite a few hundred years old, since Tolkien doesn't really mention how old he exactly is, but Legolas does refer to his companions as "children" in the novels.
The official movie guide for the LotR trilogy set him to 2931 years old during the War of the Ring.
The Hobbit takes place 60-100 years before the events of the trilogy.
And Thranduil says 100 years is nothing to an elf.
And considering that his father is in the book, it's kind of obvious that we would at least see Legolas in the movie adaptation.
They did add Legolas into the adaptation for the fangirls, but you have WAY more to complain about than Legolas.
Oh...
but..
Yeah that sounds about right. `^ ` Can't argue with that.