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.
After the 3 Stages, we have an Essential Skills module which you could check out any time you want - it's stuff that you'll find super helpful on your journey :) You'll probably find my Practical Music Theory course super fun if you're the type who likes to understand what they're doing - it's not essential for beginners but most people seem to really enjoy it!
Learn how to transition to different chords. One way to become proficient quickly is to learn how to transition between the various chords quickly. These transition periods are often the hardest for beginners to play when they are starting off. The more you practice between switching to different chords, however, the better you'll get at doing it in songs. Practice switching between open chords like G, A, E, and C. 
If you can't make the chord change from A to E in time with me, don't worry about it. Just leave the chord a little early so that we arrive together on the next chord - this also applies to all the other strum alongs in the course, and even when you are playing by yourself and counting out loud (which you should always do when learning and practicing).
When my granddaughters were young, Missy worked tirelessly to create lesson plans for my granddaughters in order to keep them interested in music. For example, if my younger granddaughter (3rd grade at the time) was unmotivated to practice in her workbooks at home, Missy would spend part of a lesson just talking with my granddaughter about what music she was currently enjoying. Then Missy would incorporate that music into her lesson plan. As a result, my granddaughter became once again motivated to practice. Missy makes lessons fun while still teaching technique.
4.) Check the guitar’s string height by pressing down on the first, second, and third fret. You should be able to do so with minimal effort. Come to the 12th fret and press down. The distance from the top of fret to the bottom of the string should be no more than three times. If it is five times, the guitar may have a warped neck or too high of a bridge.