Lots of parents have often told me that their child has learned guitar/bass/flute/clarinet....etc. quickly because they took piano lessons.
What are the transferable skills from piano?
Learning any instrument requires at least some basic reading of notes, rhythm, dynamics, and details, depending on the instrument. To know what certain symbols represent and what type of sound is suggested.
Some instruments only requires reading from one staff! While piano requires reading both the treble and bass, this gives a solid overview of the grand staff to set up the musical brain to learn any instrument.
Most instruments require the use of fingers; learning the piano allows the the student to practice finger control and learning how to use each finger independently.
Playing the piano also allows the student to develop their ear, in terms of recognizing the movement of notes (moving up vs moving down), to differentiate basic intervalic movement in sound (step, skip, leap, etc.), sounds in different register (higher, lower), and basic rhythmic patterns.
A main concept of music theory, and integrated into ear training, rhythm is a fundamental of music. To know that certain symbols mean to hold a sound longer, to play it as quick as possible, and also what symbols represent silence and how long silences should be.
Teachers of various instruments have their own age restriction guidelines because even if a child shows eagerness in learning a certain instrument, holding certain instruments with proper posture may present an issue, let alone reaching tiny fingers to cover certain keys.
Starting with piano lessons at a young age does wonders for their development and growth in many ways, to prepare for school, and also to allow a child to experience music and the world of symbolic representation.
Make sure to find a qualified teacher who fits your child's personality, and is open to trying new activities to engage your child. There is nothing worse than a negative experience stretched over years of lessons!
Happy playing! :)