Age and life stage

Method

The age is the number of days between the birth date and the calculation date, in calendar days. The birth day is P0: P63 = postnatal day 63, 63 days after birth. Weeks are whole weeks, followed by the days left.

A milestone in months falls on the same day of the month, that many months later; when that day does not exist, on the last day of the month (born January 31, 3 months on April 30).

In the mouse, the stage comes from The Jackson Laboratory: mature adult from 3 to 6 months, middle age from 10 to 15 months (JAX puts its upper limit at about 14 to 15 months), old from 18 to 24 months, very old from 24 months. Before 3 months, the stages follow the Colony.care app’s weaning and sexual maturity ages, the same values as the breeding planner. Between two published stages, the tool writes “Between stages” and invents no name. Each stage starts on its first day; the next stage’s first day ends it.

Limits

• JAX gives about 24 months as the upper limit of “old” for C57BL/6J mice, when strain-specific diseases can start to affect results; other strains may differ.
• Life stages are published for the mouse only. For other species, the tool gives the age and the app’s weaning and maturity ages, with no stage.
• JAX places sexual maturity in the mouse at around 35 days; the tool uses the app’s 42 days, like the planner.
• The tool does not convert to “human years”: mice do not mature at a constant pace relative to us (150 times faster in the first month, then 45 times over the next five, per JAX), so no single factor holds.

Sources

• The Jackson Laboratory, “When are mice considered old?” (2017): mature adult from 3 to 6 months, middle age from at least 10 months up to about 14 or 15, old from 18 to 24 months, very old beyond, after Flurkey, Currer and Harrison (2007).
• Colony.care app defaults: weaning and sexual maturity per species, the same values as the breeding planner.