Something senescent is growing old or is elderly. Senescent things are deteriorating.
Senescent and senile have something in common — the Latin root senex which means “old.” A baby or even a teenager would not be described as senescent because they’re youthful and full of life. A nursing home is full of people who could be described as senescent; they’re old. “The Who” was going to sing “hope I die before I get senescent,” but it just didn’t have the same ring to it.