Kittens need to be three months old before they can distinguish folded ears. A normal kitten's ears will start to stand up one to two weeks after birth, and can be fully erect in about a month. Some kittens are relatively weak and may take two to three months to fully erect. If the kitten's ears are not erect after more than three months, it means that the kitten has folded ears. Folded-ear cats are a kind of cartilage disease and need to supplement nutrition, especially calcium.