BTW, you notice the dfference because other parents start to get concerned, or more likely aunts of other children say something. Which was in our case that we took notice. The aunt wanted the other mothers (it was a 2 mother couple) to take their almost 2 year old to speech therapy because he wasn't even close to DD at 20 months. Or a father will get worried because his 15 month son is still crawling while Dd at 13 months was running around and throwing a stick for their setter.