Had done, have done or did

Hi! Could you please help me with such question:

By the age of 16 he had decided not to go to the university.

Or

By the age of 16 he have decided not to go to the university.

Or

By the age of 16 he decided not to go to the university.

Which one is correct?

Explain please. Can we guess from this sentence how old is he? Is "by the age of 16" a marker of time here? Should we use Simple Past?

And one more question:

Are there any differences between

a) I was working all night yesterday. And I feel tired.

and

b) I worked all night yesterday. And I feel tired.

If I say a) does it mean that I was working in some specific manner (without intermissions or something like that)? Or we cannot use Continuous here at all?

Thank you in advance.