Taken from Wikipedia:

After initial interest into degus as research subjects, degus have become popular as pets, though until very recently they were seldom found in pet shops. Their advantages over traditional small pets are their diurnal habits, bubbly personalities, the haired tail (as compared to rats and mice) and their lifespan: they are reported to live up to 13 years under ideal circumstances (though a poor gene pool/genetic background often reduces a pet degu's lifespan significantly). The average lifespan of a degu in captivity is typically around 6–8 years of age.

Some other domestic pets and their reported lifespans. Those in bold are comparisons to the degu as they're similar small pets or related to them:

  • Cat (12-14 years) - Wikipedia
  • Small Dog (15-16), Medium Dog (10-13) and Large Dog (7-8 years) - Wikipedia (the larger the dog, the shorter the lifespan)
  • Syrian Hamster (2-3 years) - Wikipedia
  • Rat (3-5 years) - Wikipedia
  • Giant millipede (7-10 years) - Wikipedia
  • Tortoise (100+ years) - Wikipedia
  • Tarantula (12-12.5 years) - Wikipedia (Although according to Wikipedia, "Males have much shorter lifespans than females because they die relatively soon after maturing")
  • Octopus (6+ months) - Wikipedia (Males die shortly after mating, females die shortly after giving birth)K
  • Jersey Wooly (rabbit - 7-10+ years) - Wikipedia
  • Rabbit (9-12 years) - Wikipedia
  • Spiny Mouse (4-7 years) - Wikipedia ("mouse" doesn't give an estimate, just says short)
  • Fat-tailed Gerbil (5-8 years) - Wikipedia

Not included: Gerbil (standard), Chinchilla, conclusive data available about lifespan on wikipedia for the list

The degu's average lifespan is probably weighted low (meaning that there are low results which offset the average and your degu can easily surpass the average) because of poor treatment and care plus the fact that they are only recently popular as pets and information about their proper care wasn't as widely established as it is now. Because of this, it is likely that degus will now tend to live way beyond their average life expectancy while the above listed pets are established and information about them has been abundant for a lot longer so these values are more likely correct.