I'm not too surprised by this. I figured they wouldn't make it for the long haul. With an age difference like that and with the woman being so much older than the man (especially in Hollywood, where looks are the #1 thing about a person), it was just a matter of time.
I DO think Demi looks older than Ashton. Although she still looks OK, she's obviously had quite a bit of work done. If you compare pictures of her when she was younger to how she looks today, she looks harsh now. It would have been better if she'd let herself age naturally. It would also be better if she put on some weight. Her stick-thin frame is adding to her harsh look.