Deciding when a child is ready for piano lessons depends on many factors. As a general rule my preference is to start beginner students between the ages of 6 and 9 years. Many teachers do start earlier (some as young as 3) and with varying (sometimes good) results!
At 6 years, a child has usually completed their first year of school, has learned to sing songs, make musical sounds with percussion and respond to music in dance, games and drama individually, and with other children. They also will have fairly well developed coordination and gross motor skills. They will also be developing fine motor skills which are important for complex tasks such as piano playing! They will usually be settled and attentive in lessons and be able to respond to an adult confidently.
I would advise parents to acquire a piano well before commencing lessons, preferably from birth! If a child grows up with a piano in the house from birth they come to piano with a strong understanding of numerous concepts which is a great advantage:
Left to Right; Low pitch to High Pitch; Graphic design of the keys (three black /two black pattern); Soft /Loud; different textures: many notes played together/single notes. Sound representing ideas; Percussive and rhythmic piano sounds, the Pedals and their effect.
Sometimes children who have grown up with a piano will have begun to create known or original melodies or pieces before starting formal lessons. This is great! The other advantage is the lack of novelty. A new student who has had a piano before taking lessons has usually reached a point where they understand that the instrument is a means to making and understanding music making rather than an exciting toy.
Sometimes inquirers ask if they can try some lessons before investing in an instrument, due to the large financial outlay. While this is totally understandable, I do not offer tuition to students who do not have an instrument. A compromise may be to borrow or rent an instrument for a term as a trial.
Keyboards versus Pianos.
I will always recommend the king of keyboard instruments: The ACOUSTIC PIANO!! Should this be impossible I would recommend a full sized digital piano with sustain pedal, weighted keys and a limited number of piano sounds. The worst option is the low-priced keyboard with several dozen buttons, gizmos and hundreds of sounds. Why? Because your child will lose interest quickly when the novelty wears off, it will never respond or sound like the “real thing” and it will become a “clothes horse” before ending up in the hard rubbish!