Ocean Majesty - 500 ish but mostly over 60s.
Azamara Quest or Journey - 694 pax = perfect.
A, n has hit the nail on the head suggesting Azamara. My wife and I were on the Quest in the Black Sea in August. Its a lovely ship, great food, especially in the Speciality Restaurants where the only suggested charge is $5pp gratuity. I thought the service from the crew was outstanding. In my opinion, you will not regret choosing Azamara. As an alternative you could also look at Oceania, whose ships are identical to the Azamara Ships. Lastly, very few children on either line. Hope this helps.
Can I suggest Fred Olsen boats for a first time cruiser? They are not to small and are certainly friendly enough. I have been on both Black Watch and Balmoral and enjoyed both. For my first time I went on P&O Oceana, it is more medium size and from what people say it is a good starter cruise. We loved every minute on it and got us well and truly hooked. I have now tried Indy, and it's the biggest and I think the best but it's not everbobys cuppa. Whatever you go on i'm sure you will enjoy it !!!