ought, aught

Ought or ought means should, in the sense of duty or obligation--
He ought to go to school.
You ought to stop littering.
Ought may be used in the sense of probability --
See who's knocking. It ought to be the delivery man.

Aught or aught ( rhymes with ought ) means zero, or nothing at all--

In the year two thousand aught one...
He can stay there, for aught I care

Now that you know that, you can say--
"He ought to be ten years old if he was born in two thousand aught two."

Note--"nought" ( rhymes with ought ) is another way of saying zero or nothing, usually British except in the old-fashioned expression "all for nought" or "it came to nought"--
He studied at night in hope of a promotion, but it came to nought.
His hard work was all for nought; they hired someone else.
Not to be confused with "naughty", meaning badly behaved, but not too--
That was naughty, little girl. Don't do it again!