Conversation so far:
What was Alexander Graham Bell's first invention?
Alexander Graham Bell's first invention was a dehusking machine made with rotating paddles and nail brushes.
Was Alexander Graham Bell's first invention successful?
Alexander Graham Bell's dehusking machines was put into operation at a friend's flour mill and used steadily for a number of years.

Question: How old was he when he invented the dehusking machine?
Self-contained rewrite: How old was Alexander Graham Bell when inventing the dehusking machine?