WebMD says...
When Baby Walks on Tiptoes
"Toe walking is common for most babies as they take their first steps. Walking on tiptoes should disappear by the time a child is 2 to 3 years old. Many babies practice walking on tiptoe as they are first learning to walk. Only later, after 6 to 12 months or so of practice, will they learn to walk with a mature heel-to-toe gait.
Usually walking on tiptoes is not a problem. But if toe walking persists beyond the age of 2 or is done constantly, see your child's doctor for advice. Persistent toe walking, or toe walking on only one foot, can be a sign of a central nervous system problem and should be evaluated"