J.W.
my 3 year old daughter took her first steps on her first birthday and never stopped. my 2 year old son started walking on his own around 13 months but would hide it from us. he would walk around by himself while playing with his sister in her room but around us, nothing...
my youngest (so far...another is on the way) is 9 months old and he doesn't act like he even wants to crawl but is trying so hard to pull himself up to stand on his own.
on the other hand, my husband's cousin is the same age as our daughter but when he was 6 months old he was already walking along the furniture by himself. i think he started walking on his own (without the aid of furniture to hold on to) by 8 months. weird to think of an 8 month old baby walking huh?
my point is that every baby is different. some babies will do it on their own and you'll never know because they aren't doing it when you're looking. others are going to start very young and others won't do anything until much later than other kids their age. if she isn't walking on her own by 18 months though, talk to her doctor. she could just be scared. she may have had a scary fall (one you may or may not have seen) or she might just be stubborn (like my 2 year old son was).
good luck though and before long we'll see another request titled "HELP ME!!! HOW DO YOU STOP A CHILD FROM BOUNCING OFF THE WALLS!!???"