There is no set age to start a child in golf; if they have an attention span and an ability to comprehend then they are ready to learn
Although many players believe that you can either putt on an arc or straight back and through, the putter shaft is attached to the head at an angle so this requires the club to travel on an arc
Many golfers can not get into a proper set up position and posture because their hamstrings are too tight to allow the straightening of the back, so before you tee off or practice work on stretching out your hamstrings so that you can work the golf swing properly around a straight back