It erupted 17,190 years ago.
It has a half life of 5,730 years. Therefore, when it has 50% remaining, it has lived for 5,730 years. When it has 25% remaining, it has lived for 11,460 (5730+5730). When it has 12.5% remaining (25/2), then it has lived 17,190 years (11,460 + 5730).
:D