senescent

senescent \si-NES-uhnt\, adjective:

Growing old; aging.

During summer, when there is no rain for months, the forests become littered with dry senescent leaves and twinges, which could burst into flames ignited by the slightest spark.
-- Dr. Nitish Priyadarshi, The American Chronicle

There is good evidence that senescent cells are not benign. But until now no one has been able to confirm that they exist in appreciable numbers in old animals.
-- John Sedivy

It was a senescent adult whose maximal growth rates were over 2.1 kg per day. Growth flattened out at around 20 years of age, and death occurred in its 29th year. Direct measurements of growth rates of T. rex and three of its closest relatives have been used to generate age/mass growth curves for these animals.
-- Nature

Senescent is from Latin senescere, "to grow old," from senex, "old."