This picture is of Audrey at about 4 and a half years of age, which is getting pretty close to the maximum normal lifespan of a guinea pig. She has a bit more white hair, and a lot more fat. She's still greedy and not that bright. These are endearing features, though.
As she's aged, she's mellowed somewhat, and will let you rub her back, though she's still jumpy about her haunches. It's almost like a on/off button - touch her back legs - she makes little angry squeaks. Stop touching her, and the squeaks cut out immediately. As far as I can tell, it's not a medical problem - she just doesn't like any physical contact other than with the bridge of her nose.
Audrey had a problem with a mouth infection that made one side of her jaw sensitive, which led her to chew and drink lopsidedly. This lead to uneven wear on her teeth, though the infection has been set right by the vet now. She still has the habit of sliding her teeth onto the drinking tube sideways, however, which has given her rounded incisors on one side.