My daughter, now 2, was born at 33 weeks. She's been either on time, early or even around 2 months behind on everything. She's across the board!
My Dr.'s rule of thumb is to watch for "emerging skills" you know, signs that they might be close to doing something and aren't quite there yet. We would also set sort of a date to look for more help if we have not accomplished a goal by a certain time.
My daughter cruised well and even would walk holding my hand, but not on her own until right at 20 months. She did not show any signs of "wanting to walk by herself" though. The key was the emerging skill piece which my Dr. recognized. And right at 20 months our intent was to involve ECI if she still wasn't walking but she came in just under the wire and just started doing it.
Once we got her comfortable holding our hand to walk, we started walking her everywhere like that. She figured that out and then started plopping her butt on the floor after a few steps and that's when it came clear to us that she didn't WANT to walk. We kept walking her though and one day we caught her walking across the living room.
20 months for my kid!!! Don't panic, but ask the Dr. if you think your child is showing "emerging skills" and what they think a good cutoff would be.