Aging and senescence are frequently discussed in the context of biological research, particularly when using mice as models due to their physiological similarities to humans. While often used interchangeably, these terms have distinct meanings. Aging encompasses the biological changes occurring over time throughout the entire lifespan, including developmental stages in youth. Senescence, however, specifically refers to the gradual decline in physiological functions that naturally occurs with advancing age. Furthermore, age-related diseases, such as cancer, heart disease, arthritis, osteoporosis, and cognitive decline, become increasingly prevalent with age. Mice serve as models for many of these conditions, prompting important questions about how their lifespan stages correlate with those of humans, especially when considering What Age Is Considered Middle Age in both species.
The maturation rates of mice and humans are not linearly related. In the first month of life, mice mature approximately 150 times faster than humans. Over the subsequent five months, this rate slows to about 45 times faster, during which mice transition through their mature adult phase. In mice, the mature adult stage spans from 3 to 6 months of age. When considering what age is considered middle age equivalent for humans, this mature adult phase in mice roughly corresponds to the human age range of 20 to 30 years.
In aging studies, mature adult mice often serve as a baseline or control group because they are past developmental stages but have not yet exhibited significant signs of senescence. It is crucial for mice in such control groups to be at least three months old, as rapid growth continues until this age, even though sexual maturity is reached much earlier at around 35 days.
After the age of 6 months, the maturational rate in mice slows further to approximately 25 times faster than in humans. Middle age in mice is characterized by the emergence of senescent changes in some, but not all, biological markers that are known to change with age. Therefore, a middle-aged group of mice is invaluable for determining whether an age-related change is a progressive development or one that manifests primarily in old age. For research purposes, mice should be at least 10 months old to be considered middle-aged, with an upper age limit of around 14-15 months. This middle age phase in mice correlates to a human age range of approximately 38 to 47 years, providing a crucial window for studying what age is considered middle age and its associated biological changes in humans through mouse models.
Mice aged 18 to 24 months are considered old, corresponding to humans aged 56 to 69 years. This “old” age in mice is defined by the presence of senescent changes in almost all measurable biomarkers across all animals. For the C57BL/6J strain, the upper limit for this old age group is around 24 months. Beyond this age, the onset of strain-specific diseases can confound biomarker analysis, leading to potentially misleading results. In very old mice, those over 24 months, survivorship significantly declines, and detailed necropsies and pathological assessments become essential for accurate data interpretation due to the prevalence of age-related histological lesions.
For researchers needing aged mice for their studies, The Jackson Laboratory provides study-ready aged C57BL/B6 males and females ranging from 25 to 78 weeks of age. These mice are maintained under a patented Genetic Stability Program to minimize genetic drift within the colony, ensuring consistency and reliability for aging research focused on understanding different life stages, including what age is considered middle age and beyond.
References:
Harrison Laboratory’s online article Life Span as a Biomarker
Flurkey, Currer, and Harrison, 2007. ‘The mouse in biomedical research.’ in James G. Fox (ed.), American College of Laboratory Animal Medicine series (Elsevier, AP: Amsterdam; Boston).
Recommended Resources:
The Jackson Laboratory has study-ready aged C57BL/6J males and females available between 25 – 78 weeks of age.
Guide: Aged C57BL/6J Mice for Research Studies
On-Demand Webinar: Research Using Aged B6 Mice – Considerations, Applications, and Best Practices
Aging-related studies in the Mouse Phenome Database
Online Aged Rodent Colonies Handbook from the National Institute on Aging
