+JMJ
I believe that the first book in the Chronicles that I read (or had read to me) at age 7 was The Lion, the Witch and the Wardrobe. My first reading of the series at that age was so long ago that I don't quite remember the order in which I read them after LWW. I'm thinking that I probably read up through The Silver Chair in publication order. However, it had for some time been my firm conviction (especially through my teenage years) that chronological order was the best way to read them (they are called chronicles after all). For one thing, The Magician's Nephew is such a magnificent story that one can hardly go wrong beginning with it. It is in fact in my top two, tied with TSC. Enjoying the backstory of Professor Digory can also set LWW in context. Yet, it is true that, as the forum member above seems to suggest, a certain element of the novelty of LWW might be lost (e.g., the mystery surrounding Aslan or the unexpected magic wardrobe that transports the children into another world). However, the shocking and scheming Uncle Andrew, the rings, the Wood Between the Worlds, Charn, and the creation of Narnia through music can be an even more striking introduction to the series. In some ways, The Horse and His Boy, even though it clearly enjoys a place in the timeline, is a standalone story that can be read at any point. It is an adventure within Narnia that involves no one traveling there from another world. So having reflected on this, I will at this point suggest the following ordering:
1. Magician's Nephew
2. LWW
3. Prince Caspian
4. Voyage of the Dawn Treader
5. TSC
6. The Last Battle
The Horse and His Boy (read at any point).