Something that I never realized is the age of fauns. I'm not sure how long fauns lived in Greek mythology, but if Tumnus can remember what life was like before the White Witch's spell was cast, he has to be over a hundred years old, right? 

While I do love James McAvoy as Tumnus in the Walden film, I think it would be a nice change to see an older actor. 

I don't think that Tumnus remembers or lived in Narnia before the Witch's rule, I think he is just telling Lucy about Narnian history, like in Chapter 8 in The Last Battle when Jewel the Unicorn is telling Jill about Swanwhite and Moonwood the hare and King Gale. He is just telling her about the past. Interesting idea, though.

@notswanwhite I always got the impression that he was telling stories from experience. Maybe not...? Good point though about Jewel telling about the past. I guess I also have Jeffrey Perry's performance as Mr. Tumnus in the BBC version in my head as he longingly describes the old days to Lucy. 

The book actually describes Tumnus as middle-aged at the end when Peter, Susan, Edmund and Lucy have all grown into adults, so C. S. Lewis must have imagined him as relatively young at the beginning. Either fauns age slowly (as Roonwit implies centaurs and unicorns do in The Last Battle) or, as NotSwanwhite says, Tumnus was just recounting history to Lucy.

I'm not sure if The Lion, the Witch and the Wardrobe actually describes the White Witch's winter as lasting a hundred years. I believe that idea was introduced by Prince Caspian. Of course, PC was written shortly after LWW so maybe Lewis had it that way in his head the whole time.

I realize this whole post has been me talking about the books and not adaptations of them so, for the record, I'd prefer a younger actor because it would evoke the iconic Pauline Baynes illustrations of Tumnus but it's not like I can't imagine enjoying an older actor's portrayal of the character. (Jeffrey Perry in the 1988 miniseries looked older than the illustrations depicted Tumnus as being and he was actually my favorite part of the cast. He had a cool voice.)

I'd personally try and skew younger with Tumnus casting, rather than older, as I think that takes away a bit of the perceived creepiness that modern audiences might have of his friendship with Lucy ... though I can equally see that some people may find a  younger casting being more creepy.

Anyway, of the various young actors who've been mentioned in rumours and chatter in the news page recently, Joe Locke feels like he could offer a fresh take on Tumnus. I've enjoyed bis performance so far in "Agatha All Along" and I feel like he could bring some of that softer friendly charm to Tumnus.

He wouldn't be my number 1 choice compared to previous suggestions, but he could potentially would be a good way to instantly differentiate the show from the Walden films.

FWIW, both the Focus on the Family Radio Theatre drama and the Walden Media movie interpret Tumnus as being centuries old. In the radio version, he prefaces his lullabye with the words, "there was this tune I used to play" (after talking about summertime celebrations in Narnia) and in the movie, he tells Lucy, "You've made me feel warmer than I've felt in a hundred years." Of course, we don't know if that was Lewis's intention. I'm fine with it though. I can buy that fauns, being fantasy creatures, age differently from humans and that a century is like a decade to them.

I've just checked the book and it doesn't specify whether Tumnus is speaking from experience of life before the Witch's winter, or whether he's talking about history from before his own time. It just says "He had wonderful tales to tell of life in the forest", and after describing some of those — in the third-person narrative voice, not in Tumnus's own words — it concludes: "'Not that it isn't always winter now,' he added gloomily."

To me, it's implied that Tumnus has experienced these things himself — we get the impression he's telling these tales with relish and regretting the fact that those days are gone, though it's not stated openly. I certainly always read it that way and was surprised when the BBC Radio adaptation of LWW (not to be confused with the BBC TV version) stated straight out that Tumnus told Lucy stories of how Narnia was in his father's time.

It's also true that Lewis doesn't specify anywhere in LWW how long the winter lasted — the "hundred years" does come from Prince Caspian. But given that Tumnus is a fantasy creature living in a magical world, I've never had a problem with the implication that he would need to be over a century old in LWW if he was around before the Witch's time, and yet he only becomes "middle-aged" by the end of the Pevensies' reign. As @col-klink said, there are other Narnian creatures that have much longer lifespans than humans (as we learn from Roonwit in LB), so it's not too big a stretch to imagine that this applies to Fauns as well. If, for instance, we assume Fauns live about 300 years on average, then if Tumnus was about 35 at the beginning of the Long Winter — young for a Faun but old enough to be able to remember things from before that time — that makes him 135 when he first meets Lucy, and then 15 years later, at the time when the Pevensies leave Narnia, he's halfway through that (proposed) average lifespan, hence middle-aged. That makes sense (in a fantasy world, at least) and doesn't conflict with anything else I can think of that we know about Narnia and Fauns.

As for how old the actor playing Tumnus in a future screen adaptation should be, I don't much care either way so long as he does a good job! But one thing I will mention (and I know I've said it before) that every live-action version so far has missed: Tumnus, canonically, has "rather reddish" skin. If they want to be accurate to the book AND satisfy the modern push for "diversity in casting", this is the perfect opportunity!!