One of the questions I am asked quite frequently is "what age should my child start learning an instrument?" Sadly there is not an easy answer to this as each child and situation are different. Let me start with the easier subject of keyboard or piano as opposed to guitar.
I have successfully taught keyboard and piano, at the same time as learning to read music, to 4 and 5 year olds. The progress is obviously slower if they start at this age, but if the interest is there, that doesn't really matter so long as they are progressing. This is of course the key (pardon the pun!) - that the interest come from the child. So long as the child has a real interest at the time and understands that it is necessary to practise at home, there shouldn't be a problem. I have found that keyboard is easier for the very young child because it can be paced very slowly. For example, with piano the student has to learn and read two different lines of music and play almost immediately with two hands. Whereas with keyboard, the playing can be restricted to right hand only for as long as seems necessary. Also young children are enticed by the different voices and rhythms and lots of buttons to press! Some may say that piano is better for the longer term approach, but one can always change over.
So to summarise, if your child is very keen and you think there is a musical ability, then middle to late Reception age is not too young if they have shown an interest in learning.
Guitar, however, is different. It is a very difficult instrument and takes much work and perserverance. I have on one occasion taken a child in Reception and on a couple of occasions children from Year One, generally because I was told the children were very, very keen. However, I still think that below year 4 is really too young and the children are physically too small! You can buy very small guitars but that doesn't change the fact that you need physical strength to hold the guitar correctly and press down the strings. When the progress is as slow as it has to be at that age and size, I question whether there is any gain to starting that young. Even at 8 or 9 years old, I feel the child needs to really understand how much work will need to be done in order to manage even a small tune.
There are - of course - always exceptions to the rule and you might ask - how do you know if my child will be that one? The answer of course is that no-one knows until they try - so there needs to be common sense and discretion applied to each individual child.