Sakurajima is not a single volcano, but a complex of three: Kita-dake (North Peak), Nakadake (Middle Peak), and Minamidake (South Peak).

While the entire complex has been active for millions of years, the current volcanic activity is focused on Minamidake, which formed about 22,000 years ago.

So, depending on what you're asking:

* The entire Sakurajima complex: Millions of years old.

* The current active volcano (Minamidake): Around 22,000 years old.