What part of South America are you considering? I don't think you will find many choices under 10 nights, particularly if you plan to round the Cape and pass through the Straits of Magellan. I did a Buenos Aires to Santiago cruise on NCL (14 nights) and then a 12 night cruise on Celebrity round trip from Buenos Aires for Carnival in Rio. In both cases the age range of passengers varied greatly (everything from 30-80). The trip during Carnival was younger overall, with lots of multigenerational families from Chile and Argentina. I know Tom did an Amazon River cruise on Oceania and was convinced he was the youngest person on the cruise (and he's a baby boomer).