My DD1 was very slow developmentally. She rolled over only a few times before a year, never crawled, wouldn't put weight on her legs, etc. At 14 months I decided to pursue physical therapy, and she finally walked at 19 months. She's 4 now and you couldn't even tell she was slow to do all these things. She is a monkey on the playground and continues to challenge herself. Some kids just develop slower than others.
My pediatrician was very worried about her lack of rolling over and crawling, but I'm glad I waited until she was past her first birthday to pursue anything.
I thought this article by Mayim Bialik was a helpful perspective on not rushing to physical therapy. It made me feel better about my own decision to wait, even if we did eventually go that route: http://www.today.com/moms/why-i-dont-force-my-kids-say-please-or-walk-1C7398514