I would vote for Regent Seven Seas. The pricing is competitive if you are looking for a Caribbean cruise and you will find plenty of 40-somethings. If you are looking for a one week med or Tahiti cruise you will find even more in your/our age group.
"Carpe diem quam minimum credula postero"
Seven Seas Explorer - Miami - Venice - Mar 2017
Silverseas Explorer - Arctic - Aug 2017
Seven Seas Voyager - Rome to Beijing - Oct 2017
Seven Seas Mariner - Lima to Buenos Aires - Jan 2018
Silverseas Explorer - Siberia - Jul 2018
Seven Seas Mariner - Australia Circumnavigation - Dec 2018