Braemar was our first cruise and we too wondered if the majority of people would be elderly. There were in fact all ages on board, including one couple on their honeymoon. Yes there were quite a few people over 65 but many of them were very energetic. On our table the age range went from about 30 to about 65. She is a lovely ship. True she doesn't have the glitz of some larger ships but the service and the food is great. I'm sure you'll enjoy it.
In our experience (4 cruises now with Fred.Olsen) the age range is a good mix - from people in their 30s to the 70 plus age group that you mention (and believe me we have met some really lively people much older than that!). The second presentation of the evening shows usually starts at 10.30 (to accommodate those on second sitting dinner). Bars are open either 'till late' or until 1.00 a.m. depending on which bar you patronise. I think that probably answers your question - have fun.
Been on the Braemar and it is mostly elderly people but not all 70plus.Not a lot of evening activity into the early hours.Having said this the ship itself is lovely and the crew as well.Would go for another cruise line as its for something special,P and O or Royal Caribbean if you want a bit of glamour.We went on it two years ago,in our fifties and did feel quite young,sorry to put a dampener on it but I wouldnt go on it again.
We went to the Carribbean on the Braemar and had a fabulous time.. there was a wide age range and we always found plenty of suitable entertainment. Believe me, you will enjoy.