prep.) The relations of time, age, or order; as, at ten o’clock; at twenty-one; at once; at first.
He came home very late at night.
The city comes to life at night.
The place at once enraptured me.
They took to each other at once.
He left school at the age of 18.
He studied botany at university.
She looked at him distrustfully.
I didn’t go, but stayed at home.
I am busy cooking at the moment.