Although OV market their cruises as '30 - 50 year olds' our experience points to your own ages being nearer the average. The age mix does depend a lot on school holidays - for a more mature age mix choose a date when the kids are in school. We have not had a problem with vibrations. Hope you have a great cruise with your family.
Stay away from the rear lower decks on the older ships in particular if vibration is your concern. The rear lower deck used to be the main ding room in its former life with P&O as Arcadia, it looks like it is now the hairdressers & gym, in choppy waters the cutlery constantly 'jingled' & you held on to your wine glass, as it hopped across the table. I would look for mid to upper decks, midship cabin.
My wife and I are in our 70's and did not feel at all out of place. There were many people of the same age bracket. We didn't pick-up any vibration problem but, in line with Wilba's advice below, we were on deck 8.
We went on Ocean Village last October and took our 82 year old father. He had a lovely time and did not feel out of place. Hope this helps.
I think the age question has already been answered (i.e. it's not an issue). With regard to cabin position I've been on OV1 / Arcadia a number of times and found that if you go for P deck (5) then you should go for a cabin no further back than P176 / P179. From that point onwards you have the engine room and associated machinery below you and you can hear the change in noise as you walk down the corridor. I had P167 on one occasion and, even though close to the movie drome, was very quiet. All you have to do now is book up! Have a nice cruise.
We have just returned from OV cruise to Eastern Med and travelled as a a family of 4 (40, 37, 11 & 9). We had a great time and found that approx. 90% of the people on board were over 50. We did not find this a problem and the children really enjoyed themselves as the pools were not overcrowded. Would definitely travel OV again even though we could be classed in the "younger" age bracket.
I agree with other comments. There is a nice mix of ages and I am sure you will not feel out of place so long as you enjoy a more informal cruise. We had previously cruised on Island Star and found we did miss the little extras particularly around food and service. Having said that our cabin was much nicer and so much quieter on OV2. We were towards the front on deck 6 and never heard a sound!