Reaching the first fret
Have the child hold the guitar while seated. The first frets should be reachable without raising a shoulder or sharply bending the wrist. If that is hard, compare a half-size model; if this feels cramped, try a larger guitar.
More than length
Body width, neck thickness and string height also affect comfort. Ask a teacher or shop to check the setup if the strings are unusually hard to press.
At what age does three-quarter size fit?
There is no age cutoff that works for every child: reach and comfort decide.