The present invention relates to a marker that can be used as aging biomarker. More specifically, the present invention relates to the analysis of nucleolar size as a biomarker for aging and metabolic health and its relation to the virtual age, or the life expectancy of animals, including humans. The aging biomarker of the invention can be used to study the effect of medication, food compounds and/or special diets on the wellness and virtual age, or the life expectancy of animals, including humans.