When Lynden was born we used different babies born around the same time as him as markers for what we "should" be expecting him to do soon. There were A LOT of babies born within 2 months of Lynden (8 that I can think of off the top of my head) so we had choices.
There was one little boy, who is just over a month older than Lynden, who seemed to be developing at the same pace Lynden was. I found myself very excited every time his mom posted something on facebook that her lil guy did because it meant Lynden would be doing it soon. It still didn't (and doesn't) stop me for worrying about how he was developing.
As I've mentioned before, Lynden took his sweet time rolling, one of the first movement milestones, and it drove me nuts. He was around 6 months when I tricked him into doing it and I was thrilled. Now I knew that he could do it, not that he continued to (much). He, honestly, rolled over a handful of times until he learned to sit on his own, then he had no need for it. He was quite capable of getting around on his back, so why bother?
I've always said (after he refused to roll) he will not do something until he knows he can do it. Very rarely does he try the trial and error way (there was very little with crawling or walking, he was quick to pick it up). He's a lot like me where he'll sit back and observe until he's confident enough that he's got it. (He also uses this approach when meeting new people, like me, so I am trying to expose him to more people so it won't take him as long).
For example, today we went to a new park (a very cool park) and Lynden didn't want to try anything. Eventually, I was able to get him to go down a slide, once, and then walk around other parts of the playground. We moved over to a smaller slide and said that Alyssa and I would go down it too if he did. After that, he was okay with going down the slide and was able to play for a bit. (Prior to this I had called my grandma to get her to give me some encouragement since Lynden just wanted to go home, and it's hard to motivate me to get him out if all he wants to do is go home. So, she offered to meet up with us at the park in hopes he'd play more, which he did.) At one point I had to feed Alyssa so I sat on a bench and told Lynden he could sit with me if he wanted. He wanted to continue to play on the playground (running across the bridges) as look as I watched him, which was fine for me. At one point I watched him as he watched an older kid go up a ladder, then another and another. By the end of our park trip he was a master at going up the ladder on his own, something he's never done before.
Alyssa, on the other hand, is going to be a much more adventurous child (and I predict more trouble). She's been trying to roll since 2.5 months, and succeed by 3.5 months. Now, just before 5 months, she's rolling tummy to back, back to tummy and all around the house because of it. She spots something she wants, usually a toy of Lynden's, and goes for it. She's also better with her hands than Lynden was at this age. Grasping things is not a problem for her and has figured out shaking certain toys make noise. She grabbed her toes sooner and is gnawing on them now, also sooner than her big brother.
She's also a very observant baby, but in a different way. Lynden is the type to watch something in particular, where Alyssa seems to be observing the world as a whole (and she seems to be very concerned).
I've heard many times before about girls developing faster than boys, and from my short experience with both sexes it seems to be true. But it seems to be more than that. Their brains work in different ways, which is not news to any woman who has lived with a man. (A good friend of mine has a daughter who is a month younger than Lynden and it's fascinating to watch them play together. Lynden is scattered and his friend is more focused. When they draw Lynden scribbles, all over the place, where she will draw circles in one spot.) It's been interesting comparing Lynden and Alyssa, in their milestones, their temperament, and how they approach things. In some cases they are similar (not many) but for the most part they are very different people with very different personalities.
Alyssa will be the type to jump in, head first, with no directions and see where the current takes her. Which is fine by me, since her brother will have looked over everything and be sitting on the shore with a boat to go get her.