Do very active toddlers calm down around age 3?

My ds has always been a complete handful. Since birth, he just wanted to move, couldn't keep him still. He walked at 9 months and never stopped. He was a bolter, and we'd take him to playcentres to wear him out but he would run around those things, climbing the inflatable slides like 50 times and after 2 or 3 hours he was still full of energy. However, recently, in the past few months, he has become much calmer. He now can sit on the bath playing for over one hour. He no longer bolts off. He can now walk for quite a while at a calm pace, whereas before he always had to move at a run. He no longer spends all evening climbing the furniture, jumping all over us and running all through the house. He now sits while we read him books and talk instead. It's quite a dramatic change and actually has me a bit concerned but a friend tells me her son also changed in the same way at this age. I wonder if a lot of boys go through this change just before 3 years of age. Has this happened to anyone else?