[Elladan and Elrohir]

Why not just take the question at face value and try to sincerely answer it? There's no need to launch into ad hominem and straw man. The guy doesn't like the movies as much as you and I do; give him a fair shake anyway and discuss his question.

It's a good question, and one that I haven't seen asked much at all. I think there was definitely an urge to play younger and give Theoden a more active role. It allows him to swing sword at Grima, power-walk the length and breadth of Helm's Deep before the battle, and go get wounded while impaling Uruk-Hai at the gate.

Of course, it also makes it more credible for Theoden to be Eowyn and Eomer's uncle; it arguably (I said ARGUABLY, mind you!) allows him to relate to Eowyn better.

And thematically, it doesn't allow Theoden to focus on his age and how that might hinder him; instead he concerns himself with boosting morale and strengthening his people.

None of those reasons are airtight by any means and all can be criticized, but those are a few that occurred to me.