noun
90th birthday
Refers to the celebration of one's 90th birthday, derived from the character 卒 which can be read as そつ and decomposed into 九十 (ninety).
祖父は来月、卒寿を迎えます。
My grandfather will celebrate his 90th birthday next month.
卒寿のお祝いに家族が集まった。
The family gathered to celebrate his 90th birthday.
From the decomposition of the kanji 卒 into 九 (nine) and 十 (ten), representing ninety.