Answer to
old chestnut:from 15 to 18:
The statement was made on January 1 of some year.
The speaker's birthday was December 31, so he/she was 16 at the time the statement was made.
On the following December 31 (of the same year that the
statement was made), the speaker will turn 17.
On the next year's December 31, the speaker will turn 18.