so big so fast

I don't know when it happened, but suddenly Emma seems so grown up. She is quickly moving from a little girl to a "kid". Does it happen when they turn 5?? Or when they start to round that corner from preschool to Kindergarten?? Here are just a few examples of things I've noticed in the past few days that remind me that my baby is getting big.
I can just barely hold her anymore....and when I am holding her, she's almost as long as I am :( She often starts conversations with, "Hey mom, you know what's really interesting.........". Yesterday, she woke up from her nap before everyone else, so she grabbed a tube and floated with me in the pool and just talked my ear off about friends and school and whatever else was on her mind. Was it really only five years ago that she was just a baby?