I think people shouldn't be so quick to disregard anyone on the list purely because of their age or appearance. There is a reason why they are on it and that's because it is possible for them to be Red John. Jane's not stupid, and unless it is a fake list ( which I doubt), they all remain viable suspects. Bruno Heller has told us he has left false clues and told a few small lies along the way, and I'm almost convinced now that Rosalind's description is one of them. Bertram remains my top suspect, but he's closely followed by Bret Stiles. I've went into a lot of detail about Gale already below, but now I'd quickly like to talk about why people shouldn't rule out Bret...

Red John's main skills and assets are pretty well known to us all by now (and, in my opinion, we should pay more attention to these, than say his hair color or his height); he's charming, manipulative, powerful, rich, mature (classical music, tea), possible ladies man, extremely sharp and clever, mentalist powers such as hypnosis, brainwashing and cold reading (similar to Jane), large following (especially, it seems, from law enforcement and Visualise).
Stiles more so than anyone ticks those boxes for me. In particular, the scene in the cell with Grace just cried out to us Red John..he came across as seductive, manipulative and very charming as he taught her how to keep herself calm and exhale out all the red smoke inside of her. What if he took a particular liking to her because she reminded him of Rosalind Harker?...could be where the "Red" in his name comes from(his attraction to red hair). It wouldn't surprise me if later on, off screen, he recruited her to his Visualise program, and possible Red John network. The main point for me though is the actors of the seven on the list. Malcolm McDowell is in a different league from the rest of them, and that could prove to be the clincher for Heller, who'll be wanting the very best to compete with Simon Baker as Jane. The majority of you will be disappointed if it does turn out to be Stiles because you say he's too old, and you may have a point. But I'd argue that Stiles looks very able bodied for his age, as in he's not thinning or stooping hunchbacked in anyway. I'd also argue that his age could possibly be a factor in why the Red John killings have slowed down in recent years. Besides, it wouldn't exactly require bodybuilder like strength to overpower young startled women (no offense) when you have a taser, a knife and a gun. Also wouldn't it make sense that he'd take back up along with him anyway, in the events of a struggle? The Red Barn episode is probably a red herring e.g who'd have guessed Haffner, now a popular choice for many, had it not been for that episode? ... So, a random man passes on the news that someone told him he saw a 'kid' painting the sign on the barn, and therefore this is supposed to be solid and categorical proof Red John is in his 40's? Come on?!?

Instead of me talking about how he has ready made followers already in his cult, which we all know, I'll finish on his relationship with Jane. No one quite manages to get under Jane's skin the way Stiles does. There's an underlying mutual respect there between the two and both are reasonably social with each other (do favors for each other) but you couldn't class them as friends. The chemistry between them however, makes every scene exciting, and would be just brilliant to watch if it turns out he has been Red John all along!
I'm not saying that he is him, just that it's still very much a possibility! Thanks guys :-)