Re: 15 month old not walking?
My oldest was crawling by 6 months, cruising by 7.5, and climbing on everything (including the very top of the couch from the floor) by 9 months. Didn't walk on her own until 15 months. My son was only about a month behind his sister on the crawling.cruising, but didn't walk on his own until 17 months. My youngest was the same as her siblings for crawling and cruising, but had actually taken unaided steps around 11.5 months and is fully walking/running (sort of) now at 12.5 months.
As said, every kid is different. Also, I think their environment plays a big role. With our son, we spent most of our day in the living room and in that particular apartment, it was such a small space, he didn't really have any reason to walk. The living room we have now is so open there's no way the baby could've cruised around everywhere.