Proper timing are words that commonly quoted by people who are full of positivity and faith. This is the same principle applied by adults as they teach any child to learn to play guitar. You cannot force them to understand everything while holding a guitar and let them fret on each string simultaneously as you wish. The proper age where they can hold a guitar is when they can hold a guitar a ¾ of ½ scale sizes, which is usually about five to six years. Some could start at the age of four but it’s a rare situation.
Nonetheless, age five is still the standard point where they could stretch their hands and holds the guitar a little better. However, this is a not a formal teaching or training lesson for kids to learn 2 play guitar because remember that they have short attention span where they cannot stay for one hour and teaching them notes. Letting them see the guitar more often, specifically in their room, may develop that interest at the earlier stages of their lives.