K.T.
I say start with the old stuff. It's easier to look back and see which photos and memories were most important and the most special... If you do something now, you will end up choosing to include a bunch of things that are special right now, and it will never end. It is too easy to get carried away. Especially with 5,000 photos.
My son is 16 months and I planned [and still plan....] on doing a baby book after his first birthday so that I could look back on the whole year. Well that hasn't happened yet either but I know it will be easier than if I had done it spontaneously as things were happening