There really isn't a best age for kids to start guitar lessons, but most children under the age of 7 generally don't have the dexterity or patience needed to learn to play the guitar. With persistence and motivation some kids have learned to play guitar at some impressively young ages, but it really comes down to is the individual attitude and maturity level of the student. 
This is a thick little book with nice big chord diagrams and shows chords in various positions. I like that it is spiral bound. The only thing missing is tabs for the letter sections so that you an easily flip to the letter note/chord you're searching for rather than having to turn a lot of pages to get to what you want. I plan to put in my own divider tabs.
Ernie Ball is the world's leading manufacturer of premium electric, acoustic, and classical guitar strings, bass strings, mandolin, banjo, pedal steel strings and guitar accessories. Our strings have been played on many of the best-selling albums of all time and are used by some of history’s greatest musicians including Paul McCartney, Eric Clapton, Jimmy Page, Slash, The Rolling Stones, Angus Young, Eagles, Jeff Beck, Pete Townshend, Aerosmith, Metallica, and more.
Unlike many string instruments which use a bow to create a sound, guitars are played by plucking or strumming the strings on the instrument, which vibrate to create a sound. Chords are played when the guitarist holds their fingers down on more than one individual string to change its length, which in turn changes the frequency of the vibration created - in other words, it creates a different note. Performers often use guitar tabs, rather than sheet music, when learning music: this acts as a kind of 'map' to teach guitarists where to put their fingers on the strings.