Gerbil, Old Age or something else?

I've had my two Mongolian gerbil brothers for probably going on 4 years now.
I take very good care of them, regular bedding changes, fresh water and
food, etc.

I know they are getting old for gerbils and I'm just curious to see if any
of you have ever witnessed death due to old age and/or what happens? One of
my little guys is lethargic and just not being himself - the other is fine,
running about, scampering if I startle him, etc. The one that is not
feeling well IS eating, I put a peanut in his face this morning and he ate
it and went for the food dish when he was done. He is breathing rapidly,
little sides moving in and out (not heavily, just rapidly like when
sleeping).

They've not been introduced to any other critters, so I know he didn't catch
something from another source, no new food brand or bedding brand or
anything like that.

Thanks in advance,
LeeAnne