Why is it das Kindesalter?

Kindesalter ends in -er. Most nouns with this ending are masculine (86% take der), but Kindesalter is an exception: das Kindesalter.

The other -er exceptions
Sense 1
das

Meaning

[1] Age of a child

[1] Alter eines Kindes

Examples

[1] "It shows two inuit in childhood who let work rest and bend their heads through a book"

[1] „Sie zeigt zwei Inuit im Kindesalter, die die Arbeit ruhen lassen und ihre Köpfe über ein Buch beugen.“

[1] "Her mother took her to Denmark in childhood and let her study music there"

[1] „Ihre Mutter nahm sie im Kindesalter nach Dänemark mit und ließ sie dort Musik studieren.“
More source examples (1)

These dictionary examples can include historical and less common uses.

[1] "He had married his cousin Luise Albertine Dörffer, who gave birth to three children, of whom the first in early childhood died"

[1] „Er hatte seine Cousine Luise Albertine Dörffer geheiratet, die ihm drei Kinder gebar, von denen das erste im frühen Kindesalter starb.“

Declension

1 Singular Plural
Nominativ das Kindesalter —
Genitiv des Kindesalters —
Dativ dem Kindesalter —
Akkusativ das Kindesalter —

Pronunciation

Dictionary entries include material from Wiktionary. Wiktionary