Care for my elderly rat

Question
Hi!  First of all, I'm not 100% sure elderly is actually the correct way to describe my rat, she's about a year and a half, but since they only live 2-3 years on average, I'm guessing a year and a half is somewhat elderly for a rat.  Her name is Ophelia and she has always had respiratory issues - she'll have periods of time where she breathes very hard and sometimes we can even hear her breathing, but she always gets better on her own.  She's also always had issues with having red around her eyes, but I read somewhere that that is a harmless condition that won't affect her, however I can't remember what source that was from so it could be incorrect.  Lately, she's been breathing hard pretty often, and she's losing weight - her "roommate", also a female (named Juliet), is about 8 or 9 months younger than she is, and she is noticeably heavier than Ophelia, whereas up until a month or two ago they were very similar in weight.  They live in a 2 foot by 2 foot coated-wire cage with a plastic bottom so they get plenty of air flow through their cage. They get along very well - I have 2 hammocks in their cage and they usually prefer to just share one; they don't fight very often and never in a very serious manner.  I'm not exactly sure of their type, they're not the very large kind of rat, I'd say they're just a little larger than a very large gerbil would be.  They've never had any tumors.  I feed them a prepackaged mix of commercial rat food, with seeds, nuts, grains, corn, etc. and I often give them cheese, apple slices or crackers with peanut butter on them.  They both came from the same pet shop and I got Ophelia when she was about 4 weeks old and Juliet when she was 5 or 6 weeks old.  They've never been bred or even had access to a male rat, since I've had them at least.  I just would like some advice or guidance on what I should be doing to make Ophelia more comfortable now that she's getting older, and what I could do to help her gain weight.  I want to be careful not to make her overweight, as I'm afraid that would make her breathing more labored, but I would like to get her back up to the weight she used to be.  I'm also wondering what I might expect as she continues to get older, if she may start having trouble getting around, or if I'll have to worry about the younger one getting rough with her, or anything else that usually happens as rats get older.  Thanks so much in advance!

Answer
Honestly,  she isnt TOO old,  though all rats will age at different speeds.  We have one that is 3 years old!!!

You should make sure the hammocks are quite low in case she falls off. You can buy a product called senior aid, which is good for aging rats (I think it's actually a dog product!) don't worry bout the younger rat hurting her, rats are hardy and robust.  You can feed her a supplement of cat food to build her up a bit,  I find this good for older rats.  

She will start gettign balder, walking a lot slower,  she may fall over a bit.  We keep our 3 year old in a one level tank, just because he cant climb,  and if he tried he might hurt himself, but dont leave your other rat alone,  we have 16 rats altogether so they all have plenty company!!  LOL