Well, the thinking is that she died (or rather, became ill) so young because of her 'real' or cellular age--which would be 12, since her mum was 6 when Dolly was cloned.
Granted, she did have a lung ailment common to older sheep--but it's also common to sheep kept indoors for long periods. Dolly spent her entire life inside a barn. Dolly was also plagued by arthritis in her rear legs and by obesity--both uncommon in sheep her age. However, the arthritis and tendency to obesity could be likewise explained away by unnatural influences--Dolly learned to stand on her hind legs and beg for snacks from visiting scientists and reporters.
So she was performing activities and living under conditions not common to the life experiences of other sheep; I guess I find it hard to believe that these experiences didn't contribute more to her early demise than her being a clone did. Not that I'm an advocate of cloning--I'm not, but I do think Dolly was a rare success.
They're going to be doing a postmortem on Monday, and then Russ can have his mutton. Mmm, mutton!