My wife and I sailed on the Nautica from Lisbon to London last summer. We are both in our late 50's. We had a wonderful time and will definitely sail on Oceania again.
You will definitely be on the young side compared to most. However, I think you'll find that to be the case on most, if not all, Med cruises. There were certainly some passengers in their 30's.
We've sailed on many other cruise lines in Europe and elsewhere and found Oceania passengers, overall, to be a little younger compared to many other lines. I don't think it will be a problem.
It will be very hot and more crowded in ports in August. We much prefer May when temperatures are milder and things aren't as crowded. An excellent site for weather anywhere in the world by month is http://www.weatherbase.com
If you have any other questions about the ship or the line, just ask.