The awaited Kalki avatar could be young like Lord Krishna or a fully matured grown person, one possessing jnana wisdom of Bhagavad Gita and Upanishads (independent spiritual treatises).
In present times the age of Kalki is immaterial, what matters is the work done by Kalki avatar.
The prime purpose of advent of Kalki avatar is re-establishing Dharma (righteousness) against all odds. If as a child Kalki avatar succeeds in doing so, then Kalki avatar could even be a child.
Most likely Kalki avatar would be a matured fully grown person, a man from northern India.
The possibility of yester years Swami Vivekananda manifesting form of Kalki avatar in present times is not ruled out. Swami Vivekananda in his lifetime failed to reach state of self realization, god realization. Swami Vivekananda had all the facilities of becoming a self realized person in his lifetime but he did not want to.
The goal of life for Swami Vivekananda was community service and in that he excelled. He left realization of god, realization of self for next manifestation.
The coming times would tell who shall be Kalki avatar! Only when the society reaches a point of no return shall Kalki avatar surface.