Hi Gwen,
My DD is 20months, 17 monts corrected and is still not walking. We see a physio who is helping her atm - but she has to get orthotics to help with her stability due to lack of muscle tone (common in premmies). Getting your DDs hearing checked is the best way to go and if you are concerned about walking perhaps see a physio. As stated above, your pead. should be able to let you know if you should be worrying and what the best steps to take are.
Best of luck.