walking at

  1. Children usually start walking around one year.
  2. Children usually start walking at around one year.
    Which one is correct?
    Why we have to use ‘at’?
    Please explain. Thanks.

Strictly speaking, I would not prefer ‘at about’, ‘at around’ etc. We need to be either specific or approximate, not both at a time, though even native speakers might use it so.

You obviously know that the second is correct as otherwise you would not ask why we have to use ‘at’.
In this example it helps indicate the approximate time.

Compare with:
I will come to fetch you at three o’clock.

‘at’ relates to the time.
It would be incorrect to write:
I will come to fetch you three o’ clock.

‘Why we have to use ‘at’?’
Is this question correct?
Why do we have to use ‘at’?
Is this question correct grammatically?

In the given example, an approximation is as close to a specific time as you could get. Therefore the use of ‘at’ is very relevant.

The second question is the correct one.