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.
The two most common types of acoustic guitar are the steel-string guitar and the classical guitar. The classical guitar, also known as the Spanish guitar, usually uses nylon strings. Steel-string guitars originated in the United States and are also referred to as western guitar and folk guitar. Steel-string guitars sound louder and brighter than classical guitars, which have a warmer, mellower sound. Steel-string models usually have larger bodies and narrower necks, and they often have a pickguard to protect the body against scratches by picks and fingernails. Most acoustic guitars have six strings, but there are also 12-string versions. When an acoustic guitar is hooked up to an amplifier, it is referred to as acoustic/electric. Some acoustic/electric guitars incorporate a cutaway design, which makes it easier for electric guitar players to cross over to acoustic.
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.