It very much depends on which instrument parents plan to let their little ones learn.
For piano, you can start playing it as early as 2+. Different children have different pace in their fingers' muscle development. Parents have to understand the fact that piano playing involves quite a complicated procedure. A child is only ready to play the piano when he/she can separate their fingers individually. Parents can help by encouraging them to count with their fingers, using both right and left hand. Music for the Intelligent Mind Playtime does not encourage too much piano playing until after half a year of the course training. Within this half a year, we encourage the use of fingers, such as playing with dough, counting using fingers to activate the fingers' muscle. Thus, these activities would prepare the children better when they are encouraged to play a simple tune during the 2nd half of the training.
For violin, we encourage students to start off with Music for the Intelligent Mind Playtime first to develop their pitch, rhythmic and harmonic sense first, before they start with violin playing at the age of three. There is no fixed pitch on a violin as on a piano, so it is good to let children develop the hearing first before they start playing the instrument. We do not find the usage of stickers on the fingerboard help them too much in their hearing and note reading. Recently, several researchers have started to write their thesis again about developing perfect pitch. It is proven that children who start to recognize pitches especially by singing under the age of three find it easier to develop perfect pitch.
For guitar, the approximate age to start is six.
For vocal training, you could start training seriously at the age of twelve and for flute, it is approximately at the age of seven depending on the build of the child.
For tin whistle its about five years old.
And lastly, to answer this question, I'd like to quote from one of the leading music educator of the last century, Zoltan Kodaly, "A child who plays before he sings may remain unmusical for a lifetime" "See what you hear and hear what you see" That is why sometimes I hear parents and students making such a comment: "I've practised very hard for the exam but why do I get such low marks for my playing?" The quote from Kodaly above answers all!!