。
Grandpa has become quite frail from old age.
Literal
Grandpa [subj-が] considerably aged-decline [progressive-している].
老衰 'senile decline / age-related frailty' is a formal/medical compound (老 'old' + 衰 'decline'). かなり 'considerably / quite' marks the degree. ~している is the progressive/current-state form. A straightforward medical-observation sentence.