1.
moon age; lunar age; number of days since the new moon
The number of days elapsed since the last new moon, used to describe the current phase of the moon.
2.
age in months (of a baby or infant)
The age of a baby measured in months rather than years. Commonly used in childcare and pediatric contexts.
A word with two distinct usage contexts: astronomy/nature and childcare. The astronomical sense is important for fishing, gardening, and traditional calendar activities.
SENSE 1 — ASTRONOMY:
月齢 counts the days from the new moon (新月, 月齢 0). A full moon is approximately 月齢 15. Fishermen, gardeners, and tide watchers use this information regularly.
SENSE 2 — CHILDCARE:
In Japanese childcare, babies' ages are often given in months (月齢) rather than years until they turn one or two. Developmental milestones and vaccination schedules are typically described by 月齢.