According to Shroeder's Tinys & Toys, the age that Chihuahuas' ears stand up is a matter of individual variation. In fact, some puppies' ears go through alternating periods of droopiness and perkiness until they remain upright for good.
Often the ears on Chihuahuas do not stand up until they are two to three months old, states Shroeder's Tinys & Toys. Occasionally, some newborn Chihuahuas have erect ears before they are able to walk. It is unusual, though not impossible, for a Chihuahua's ears to remain floppy throughout its life. The ears of this breed also vary in size, from small to large. In addition, some Chihuahua puppies have ears that curl back at the tops, but these curled ears tend to straighten out as the animals age.