Most children do tiptoe walk when they first start walking on their own, but if it persists then the child should be seen to rule out any physical problems. Some children will tiptoe walk well into early childhood because their achilles tendons are too tight, and if you wait until they are of this age, then they often require serial casting to bring their feet back down. Tiptoe walking throws the knees, hips, back & shoulders out of proper alignment and can cause significant pain and/or discomfort, as well as set a person up for injury later in life. With a 21 month old, It is generally not a huge concern yet, but you could spend time stretching his calves & ankles for him to help him and maybe prevent the tiptoeing from becoming a long term problem.
Also, children who have an Autism Spectrum Disorder also tend to be tiptoe walkers, so if he is showing other signs of this disorder, see your pediatrician.
at 3:08 PM on Mar. 5, 2010