Conversation so far:
How did the Australian Mining Corp start?
David used his advocacy and organisational abilities to set up the Australian Mining Corps.
What year did this happen?
August 1915.
Was this a military endeavor?
David wrote a proposal to Senator George Pearce, the Australian Defence Minister, suggesting that the government raise a military force to undertake mining and tunnelling.

Question: Do you know how old he was when this happened?
Self-contained rewrite: Do you know how old David was when David wrote a proposal suggesting that the government raise a military force to undertake mining and tunnelling?