This 2016 German human study found:
The current work employed the frailty index, a multi-dimensional approach that combines  parameters of multiple physiological systems and functional capacities. The present findings were based on [1,820] study participants aged 50 to 75 years.
Innovative approaches like Mendelian randomization will be needed to elucidate whether epigenetic age acceleration indeed plays a causal role for the development of clinical phenotypes.”
The study had an informative “Age acceleration and telomere length are not correlated” section with references. It was another step toward establishing the Horvath epigenetic clock for widespread usage.
http://clinicalepigeneticsjournal.biomedcentral.com/articles/10.1186/s13148-016-0186-5 “Frailty is associated with the epigenetic clock but not with telomere length in a German cohort”