Its hard to say how long red eyes live, because red eyes haven't been captive bred long enough to get enough personal accounts. Most accounts of lifespan are from wild caughts. I have heard a typical lifespan of a wild caught one is about 4-6 years. If they are captive bred babies, and you give them good food and proper housing / cleanliness, I would guess they could live 7 years or more. I talked to somebody that knew a person that had theirs for that long.
Concerning your skinny baby, just try to feed it. Keep it separate from the others and keep it warm, at least 80-82 degrees during the day. It may just be skinny and weak because it hasn't gotten enough to eat with bullying from bigger siblings.
If that doesn't help, contact your local veternarian and be willing to take him in.
pitcherplant7
Currently,
2 clawed frogs
2 firebellied toads
1 D. tinctorius Surinam cobalt