The frailty syndrome describes an old person whose reserves have shrunk so far that a minor stressor causes disproportionate decline, and the engine of this process is loss of muscle. $Sarcopenia$ $-$ the progressive fall in skeletal muscle mass, strength and function $-$ produces the grip weakness, slowed walking speed, fatigue and inactivity captured by the Fried phenotype, so it is the most direct determinant of frailty. The other choices are weaker: chronological age correlates with frailty but the two are conceptually separate, since fitness varies widely at any age; frailty is in fact commoner in women, ruling out male gender; and gait speed is merely one downstream marker rather than the cause. Therefore the underlying answer is muscle loss.
\[\boxed{\text{Sarcopenia}}\]