If a person is 11 years old this year how many days old his he?

It depends on their actual birth day. If they were born on December 15th, then you would simply multiply 365 by 11. This does not take into account leap years though.

If they were born on November 30th. then you would multiply 365 times 11 plus 15 days. The 15 comes from Nov 30th to Dec 15th.