Sheila appeared as a child in the episode "Friend or Foe." She was short and green and also had cotton candy and a blue skirt. She was seen when Plankton floated away, holding a balloon. Mr. Krabs is looking up in the air and Sheila is seen as a kid. Sheila is probably around 18, but oddly, she was alive when Mr. Krabs was a kid, or the young version of her seen could be another family member.
"Finally. A cool place for teens to just, you know, hang out. ("Bossy Boots")
Sheila appears to be a giant squid and is friends with Pearl, a sperm whale, which is ironic, since both creatures in real life have a predator/prey relationship.