get on in years Someone who is getting on in years is growing old.
My grandmother is getting on in years. She's no longer able to look after her home without help.
knee-high to a grasshopper This term refers to a very young and therefore small child.
Look how tall you are! Last time I saw you, you were knee-high to a grasshopper!
long in the tooth A person who is long in the tooth is a bit too old to do something.
She's a bit long in the tooth for a cabaret dancer, isn't she?
mutton dressed as lamb This expression refers to a middle-aged woman who tries to look younger by dressing in clothes designed for younger people.
The style doesn't suit her - it has a mutton-dressed-as-lamb effect on her!
no spring chicken To say that someone is no spring chicken means that they are quite old or well past their youth.
How old is the owner? I don't know but she's no spring chicken!
over the hill If a person is over the hill, they have reached an age when they can no longer perform as well as before.
Gran! You say you're over the hill, but you're a super cook!
rob the cradle If you rob the cradle, you have a romantic relationship with someone who is much younger than yourself.
My uncle Ted is dating a twenty-year-old girl. That's really robbing the cradle!
silver surfer A silver surfer is an elderly person who uses the internet.
After just a few questions my grandmother was ready to join the silver surfers.
put years on If an event or difficult situation puts years on someone, it makes them look or feel much older.
I hardly recognized Mr. Brown. His illness has put years on him.