If the answer is "by their age" the ending of the sentence needs to be "oleh umurnya" "oleh umur mereka".
No, this can be general in Indonesia
edit: but warga 62 kaum rebahan like to go w the shorter option unless in cases where clarity is ABSOLUTELY required
"They are weakened by the age" shoukd be OK?
no bc that refers to the general era instad of a particular group of people