My dog Pixie is now 5 months and they finally stand up. It is important I think to have a groomer or you can do it trim the hair on the ear so that there is no hair hanging off the ears.
Jon-An's Yorkshire Terriers (How to train Yorkies ears to stand) This site helps. the cartlidge is forming until 6 months of age so it is important to get the weight off (hair) bc it weighs the ear down.
Pixie had 1 ear up at 3 months and at 5 the other one popped up.
Also teething their ears will droop!