I think their target demographic is Millennials; even the language of their advertising, and email marketing seems targeted that way.
That being said, I think that if you are flexible and open then anyone can enjoy a VV cruise. If you are locked in to the traditional style cruise or if you aren't open to new experiences, you may be disappointed. We met two different couples in their 60's, one was enjoying their cruise and the other was very negative. So more than age, to me it's a matter of attitude.
The rooms are very minimalist which suited us fine. There was plenty of storage. Our sea terrace bathroom was small but the shower itself was larger than most cruise ship showers.