P-H ..... the answers are even far more varied than your questions... Some'one-man-bands' make their living at it while others play as a hobby.... some have VERY elaborate PA set-ups, some - one amp .... party size - intimate to 300 - 400 people....Ages? they run the gamut....
A visit to the S-Z 'General Arranger Keyboard' page will give some insight....
t.