Diplomatically speaking - you have booked a 3* cruise line so you will get the same sort of clientele that frequents a 3* hotel. Have you been over to cruise critic boards to read what people there say? It is not very active as FO appeal to and market to a British audience [which is another downside...IMO]. Americans are generally more 'lively'.
Well the passengers I sailed with on Fred Olsen seemed a pretty 'lively' bunch, and I was a mere 33 years old...
You make your own entertainment. It won't be a party ship, but it won't go to bed at 10pm.
FO do have a reputation for 'older' passengers, but they're not all past it. I'm sailing with them again next year, and have no worries.
I've sailed with several cruise lines and do not consider Fred Olsen to be 3* in any way at all.