expression, suru verb (irregular)
die a natural death at an advanced age; live out one's allotted lifespan
Idiomatic expression used when someone dies peacefully after living a full life, without accident or illness cutting it short. Often used in obituaries or respectful descriptions of a long life well lived.
祖母は九十歳で天寿を全うした。
My grandmother lived out her natural lifespan and passed away at ninety.
彼は長い闘病の末、天寿を全うするように静かに息を引き取った。
After a long illness, he passed away peacefully as if having completed his allotted years.