When the Rover section first began in 1918 in the UK, it followed straight on from the Scout section. It was in the '40s that Senior Scouts were introduced between the two. In the UK Scout Association, the Senior Scouts section's name was changed to Venturer Scouts in the late '60s and other countries such as Australia, Canada and New Zealand followed that path. I believe the US equivalent were Explorers.
When the Baden-Powell Scouts Association first emerged in the UK in the early seventies, they retained the traditional section names of Senior Scouts and Rovers.
In the (WOSM) Scout associations of many countries, Venturers (or Venturer Scouts) and Rovers are still part of the structure today. Age range varies, but for Rovers is usually 17 or 18 to 24 or 26. Originally, the Rover section had no leaving age - "Once a Rover, Always a Rover"!
If you're interested to find out about the Rover section in Scouts Australia, visit http://www.rovers.com.au
A good history of the Rover section can be found at http://www.scouting.milestones.btinternet.co.uk/rovers.htm(This message has been edited by Nick B)