We use at to talk about points in time, ages and some periods of time:
I was up at 6 am this morning. (a point in time)
At 12 noon, all the bells rang out. (a point during the day)
They all get dressed up at Halloween. (a point on the calendar)
Many children leave school at 16. (referring to a specific age)
At 20, I thought I knew everything.
We can use at to refer to some periods of time: at night, at that time, at the New Year:
They leave the front-door light on at night. (period of time)