Personally I think cruise line age demographics have blurred considerably in the last few years. Mainly this, IMHO, is down to the competitive way that cruise lines market themselves to every age group. At one time you could assume that Cunard were for the older generations but not anymore...QM2 and QV have made Cunard far more appealing to the younger crowd and families. In the same way, Carnival have embraced all age groups and don't aim specifically for the younger age groups which they had to do, in a way, when they started up in order to encourage those age groups to cruise and to convince younger people that cruising isn't just for old fuddy-duddies.
Even the higher end of the cruise lines...Regent, Oceania, Crystal...are actively encouraging younger and younger guests.
I think you will have a wonderful time on whatever ship you choose...age is but a number and the cruise lines have finally cottoned onto that...which is a good thing in my book
Every cruise I have done in recent years has had age 5 thru 95 on it and everyone had a great time mixing with each other and enjoying what each ship had to offer