That's true. I mean, good grief, Hayden Christensen (Anakin Skywalker, though not much of an age difference) was playing a nineteen year old kid, yet he was like 22 or 21; and Natalie Portman in Ep 3 was playing a 27 yr. old, yet at the time, was only like 22 or 23.
22 or 21 and playing a 19 year old is much, much different than a 25 year old playing a 14 year old. And a 22 year old playing a 27 year old is perfectly acceptable.
I'll have you know that the guy who played Pippin was about 25 when they wrapped filming (not sure of his exact age, but he wasn't exactly as young as his character) so what is the big deal?:
So.. if a young person of, what, barely 18, as you said? Can play a near 40 year old man.. why can't a near 25 year old man play a teenager?
With regards to the hobbits in LOTR:
1. They are never meant to portray humans. Hobbits age much more slowly than humans. Plus they live in the Shire, which is this perfectly idyllic, peaceful place. They are going to look younger for far longer.
2. That being said, they are never portrayed as young hobbit boys. There were young hobbits; they looked like normal children, around at the beginning of the Fellowship of the Ring to show you clearly their society. The 4 hobbits in question were all young men, a time period which lasted until 60's or 70's for hobbits.
Now WHY do we have Ben Barnes playing Caspian, a person who was a young human Telmarine Boy in C.S. Lewis' Prince Caspian? Why was he even considered?
That is the question. But I think what his selection means is that:
1) Disney clearly intends to stray far from the MEANING and TONE of the original work. The PLOT may stay the same, but other things will change. No longer will Caspian be this wide eyed, innocent, longing youth, but rather a more jaded, man-of-the-world, action oriented, Ulysses type of character. This characterization will strip out much of the meaning and tone from the original work.
2) With Barnes, it can't help but be a darker and much more serious work than the original. In the History Boys (and I saw this play on Broadway) he played Dakin, a manipulating, bisexual 6th former with a huge sexual appetite. Now that’s fine, his character correctly portrays many late teens. And I liked the play a lot. But someone that can play such a character should not even be able to play the boy Caspian as envisioned by Lewis. The boy Caspian knew nothing of such things.
So the tone and the meaning of this classic work are going to suffer. Where were Douglas Grisham and others responsible for maintaining the integrity of Lewis's work when this selection was made? Probably hoodwinked and browbeaten by the intimidating "creative" forces at Disney.