Maybe the person that told you is getting info about guinea pigs mixed up with chinchillas? Female guinea pigs, if not bred before 2 years old, can't be bred. More specifically can't give birth, since their pelvis doesn't widen enough and will likely die during birth.
As far as I can tell female chinchillas can breed pretty much until they die. So they can physically breed anytime from about 8 weeks to 20+ years. Though I'm not saying breeding that young or that old is a good idea though.
You can try getting another female chinchilla, but there is no guarantee they will get along. Even having a playmate, or just another chin in the same room to chat with can help making them not feel they are alone. If that option is out, then if she doesn't chew fleece, you could try giving her a cuddle buddy, it's basically just a chin size and shaped stuffed animal to cuddle with.
Personally I would not house a degu and chin together, their personalities often don't mix, I think you just got lucky. I'm also pretty sure that their diets, temp requirements, and sleep cycles are not exactly the same. Chins are from a higher altitude Andes then degus, so are use to a drier, colder enviroment. In addition, degus don't live as long as chins, 5-10 years instead of a chins 15-20 years. So the chin will be faced with loosing it's companion at least a couple times, and that is very stressful and depressing for chin.