We have cruised with P&O a few times since my Coeliac diagnosis 10 years ago. We're now in our early 40s so certainly not old fogies or care-home material (yet!). We have been with my sister and her husband (similar age) and also my parents (in their 70s) and have shared a table for dinner and had a lovely time. So far we've only been on Arcadia, which is apparently known for an older crowd than some of the other ships. I will admit that grey hair dominates but we certainly weren't the only younger people onboard and met many other parties, like us, with a Coeliac person and cruising as an easy and safe way to travel abroad. From what I gather, Britannia is generally a younger crowd anyway.
As others have mentioned, at dinner in the evening you will be presented with the menu for the following night (and also for lunch if you're likely to be onboard then as opposed to exploring ashore). The Restaurant Manager will personally take care of your requirements. The one that we had on Arcadia in August was just wonderful - nothing was too much trouble and on a couple of occasions he got me something which wasn't actually on the menu at all! - at lunchtimes P&O used to do some excellent curries but these were sadly lacking this time so I enquired and duly got a delicious curry for the following lunch! On one occasion we had decided to eat in the speciality restaurant, Sindhu, and mentioned this to the restaurant manager and he booked our table for us and specified my allergies so that everything was sorted when we turned up the following evening.
On the first night of course you can't have booked ahead so you just need to let your table waiter know and very likely have one of the "everyday" dishes - there are always basic chicken, steak and salmon available if I remember rightly. The restaurant manager will seek you out during the course of the dinner.
I had ice creams several times and was never told that it wasn't GF (and didn't experience any problems).
I will admit that I don't think that the food is quite as good as it used to be - there is a little evidence of cost-cutting in comparison to 10 years ago, but we still found it perfectly good and I felt safe eating it. There were also a few occasions when my dish was a bit different from the "normal" equivalent - like my vegetables would be different which was a bit disappointing but I suppose that's to ensure that everything is prepared separately.
I didn't eat in the buffet at all as I found it easier and less stressful to order all meals in the main dining room. It's also less of a bun-fight!
Regards the dreaded norovirus (ugh) I have had that onshore twice but so far (touching wood!) have managed to avoid it on cruises. Probably using the main dining room instead of the buffet helps. Also we make sure to use the hand gels, take our own anti-viral gel for when out and about (I am a bit of a germophobe!) and we also use our own cabin loo rather than the public ones on the ship.
I hope that this helps and I'm sure that you'll have a lovely time. We have booked for P&O Aurora for spring next year and hope that it will also be good.