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. 
BUT, I use it a very differently. I use a week's lesson as a week's lesson! I don't play one a day, I play the whole week's worth each day! And, I stay on the week's lessons until I have them down. I very often back to earlier weeks, and go over them to see how much better I play them, see if they are really in my fingers..and they are perfect warm ups. I go on to new week's lessons, only if and when I feel ready.
For the longest time I have "messed around" with my guitar. I bought a book on Jazz guitar chords many years ago and found that those fingerings were way above my skill level. I decided to get a basic chord book to reinitiate myself. This is that book. Going through these pages has re-ignited my lust to learn more about playing my guitar. One chord at a time. Simple. Skip to another key just as simple. Actually, I haven't realized how many chords I really know how to play. I just need to learn the name of those chords. This book will help with that too. Give it a try.

In music, a guitar chord is a set of notes played on a guitar. A chord's notes are often played simultaneously, but they can be played sequentially in an arpeggio. The implementation of guitar chords depends on the guitar tuning. Most guitars used in popular music have six strings with the "standard" tuning of the Spanish classical-guitar, namely E-A-D-G-B-E' (from the lowest pitched string to the highest); in standard tuning, the intervals present among adjacent strings are perfect fourths except for the major third (G,B). Standard tuning requires four chord-shapes for the major triads.
Open tunings improve the intonation of major chords by reducing the error of third intervals in equal temperaments. For example, in the open-G overtones tuning G-G-D-G-B-D, the (G,B) interval is a major third, and of course each successive pair of notes on the G- and B-strings is also a major third; similarly, the open-string minor-third (B,D) induces minor thirds among all the frets of the B-D strings. The thirds of equal temperament have audible deviations from the thirds of just intonation: Equal temperaments is used in modern music because it facilitates music in all keys, while (on a piano and other instruments) just intonation provided better-sounding major-third intervals for only a subset of keys.[65] "Sonny Landreth, Keith Richards and other open-G masters often lower the second string slightly so the major third is in tune with the overtone series. This adjustment dials out the dissonance, and makes those big one-finger major-chords come alive."[66]
Learn a D major. This chord only requires the bottom four strings. Place your index finger on the 3rd string, 2nd fret. Your ring finger then goes on the 2nd string, 3rd fret, and your middle finger is the 1st string, second fret. You'll form a little triangle shape. Only strum these three strings and the 4th string -- the open D -- to sound out the chord.

My personal opinion on the topic, as one musician to another, is that the best thing you can possibly do when trying to figure out which string to go with is to try out as many different brands and types of guitar strings as you can. Strings are cheap enough that most people are going to be able to afford to experiment, and the truth of the matter is that you’re probably not going to really know what works best for you until you have hands on experience.
SFCM produces some of the most successful and influential classical guitarists in the world. Spearheaded by renowned faculty and complemented by visiting artists such as Marcin Dylla, the department honors the tradition of the classical guitar while cultivating innovation. The Harris Guitar Collection, housed at SFCM, gives students a chance to see—and play—some of the most extraordinary guitars of the last two centuries.