Everyone ages. Soft skin goes loose and rough. Baldness occurs later in life if you have the gene. Pounds are added as life increases. All these can be factors that have altred the view of Red John in our eyes. It's perfectly natural to age over a ten year time span of mass killings. I don't care how good you are, if the stress doesn't get to you right away, it will show in your body later in life.
That being said, is it possible we are all looking at this from the face value, present time frame? What if the descriptions we've been given of Red John by his discriples are those that occured years back and not now? We have no way of knowing if Red John dressed up or altered his appearance for Rosalind Harker...after all, she is blind. She wouldn't know the difference. Occam's Razor I feel applies here. The simplest explanation is likely the most correct. We are basing our facts about Red John of possible red herrings, faulty memories, inreliable witnesses or accomplices, and not the simplest explanation. What if Red John is none of these? What if he dressed differently when he saved PJ at the end of Season 2 than when he would videotape/spy on CBI and send them footage, or for Rosalind Harker? Do I have the answer? No. I'm just trying to figure it out like all of you and discern the red herrings from clues.
Some things I think we should think about, since I haven't seen them brought up in many theories lately.
1) What about the last episode of season 5, where Red John said he'd be killing again, often, until he catches PJ or vice versa. Where are all these killings? We have no way of knowing if RJ actually killed Kira, he didn't slash her like usual victims. Lisbon was saved when Patridge was killed. I expected more classic RJ murders. Where are they?
2) How the hell did RJ read PJ's mind about the childhood memory? This still has not been touched on. At all.
3) RJ is getting sloppy. Too sloppy for someone who knows he's being hunted. I would have thought he'd be more careful since any slipup is a clue for PJ. And we know how pissed RJ was when he found out PJ learned of their handshake. And that was one clue after ten years.
One last thing. Christopher Nolan vehemently denied any rumors or speculations about Ra's al Gul's daughter being Talia in the Dark Knight Rises. What happened there? I'm not prescribing to the Patridge-not-dead theory, or the twin theory of the Kirklands, or the RJ-is-not-on-the-list theory...I'm just saying that anything goes. And Heller will intentionally mislead us the way RJ or PJ does. Was Bertram not bald at some time? Did Stiles look substantially younger 10 years back?
I'm hoping that the reveal will not be a let-down and all our questions are answered. A nice, clean closure a la JK Rowling's final Potter book would be excellent.