Cat's third eyelid showing?

Is this normal? She is 11 years old.

If it's unusual for your cat, it's a sign that you need to schedule a vet appointment.  The 3rd lid is sometimes briefly evident in cats when they're falling asleep or exceptionally contented (like when they're being stroked), but this is a trait present from kittenhood and not something that occurs spontaneously.  You definitely shouldn't see the lid on a wide, alert eye like the one in your pic.  

Cats are talented at hiding pain and illness, so we have to pick up on subtle cues like this as an indication that something is off.  And it's especially important to watch for any changes in appearance or behavior once they enter their senior years, since they're more prone to illness.  

Best wishes and keep us updated!

• Third eyelid  protrusion:  If the third eyelid becomes visible or crosses your cat’s eye, he may have a wound or may be suffering from diarrhea, worms  or a virus.

If you notice that one of your cat’s third eyelids is covering one eye more than the other, it is likely that your cat has injured that eye.  Eye injuries are painful and can become serious quickly, so you should seek veterinary care right away.

when cats are in a deep sleep or have been given a sedative, the third eyelids can become prominent.  If your cat has lost a lot of weight, the fat pad behind the eyes may also have decreased in size, changing the position of the eye in the socket and allowing the third eyelid to become visible.  Rarely, inflammation due to a neurologic, respiratory or intestinal infection can affect the nerve control of the third eyelid.

is your cat dehydrated? 

One way to assess hydration in an animal is to lift the skin over the animal's shoulder and watch how fast it goes back to its normal position. In a normal, healthy animal, if the skin between the shoulders is lifted up and then released, the skin will pop back to its normal position immediately.

In dehydrated animals, there is less fluid in the skin and it is less elastic. When lifted off the back, the skin of a dehydrated animal will not immediately fall back to its normal position. If a pet has lost 6-8% of its normal fluid, there will be a definite delay in the skin returning to its normal position. If the pet is 10-12% dehydrated, the skin will actually look like a tent and not go back to its normal position. Signs of shock may be evident. If a pet is over 12% dehydrated, it is an extreme emergency.

Other ways to assess dehydration are to examine the mucous membranes  (gums); they should be moist. In a dehydrated animal, the eyes may appear sunken in. In very dehydrated animals the heart rate may be increased, but the pulse would be weak.