I suspect the milestones thing is more a matter of birth weight and how well they eat. I've got an intense baby who just turned 10 months and is literally running around the apartment, trying to climb the furniture, signing a dozen or so words, following simple instructions, and biting when you make her angry. Of course she has consistently been ninety something percentile in weight. My wife is constantly amazed. The babies in her family are generally very chill, but ours hates strollers/carseats, refuses to nap more than an hour a day total, wakes with night terrors every hour or two, and screams/chases if her mother gets more than about 5 feet away. Each one is different I guess.