I just noticed the actual title of your thread Ann! I think I was extremely fortunate to have my "most G" kid last. We were able to take advantage of our own "hindsight" from the other two. I laugh now to think that I started DD13 early primarily for her height, but it was truly the right move across the board and helped pave the way for DS a few years down the road.
But now with real hindsight....what might I have done differently? One lesson I learned is that schools will simply NOT plan for the uncertain thing. Talking to them about what they might do in 2 years is really somewhat pointless, sad though that is. I would be more "forceful" in the here and now, during those "now gone" moments that would have possibly made a difference.
I would also be less wishy-washy in those early years. One of the best things someone ELSE did for me then was when DS's preschool teacher tore up his check for the second year. He was double booked there and in a private K, and I was really hemming and hawing. Had she behaved more like the typical school administrator, he surely would have done the second year there. If a school admin senses insecurity on your part, your child will most certainly be doing the "status quo" which may or may not be the right thing for your child. I've found boards like this to be the best place to really "talk it through", and then I'm able to give the illusion of confidence with the school. This has been needed for us as DS is somewhat off the beaten path at this point (2 years accelerated, 2 more in math). Schools don't really want to discuss a child, they want to make decisions and move on.
And YES....absolutely track those milestones! Perhaps you learned that lesson from me, who has lots of regrets in that area. At a minimum it's such a lovely keepsake for YOU! I'm sure you've heard other older parents say this, and perhaps like me...you don't believe it, but you WILL forget. A lot!