Guitars come in many guises:
Classical guitar (playing traditional guitar music)
Acoustic guitar (playing mostly chords and solos)
Electric guitar (playing mostly chords and solos)
Bass guitar (playing bass lines)
Classical guitar can be played as early as year 2. A good age to begin the electric guitar or bass guitar is year 5 or 6 upwards.
The Ukulele can be learned by pupils as young as year 1.