If you're 18 to 25 and still want to enjoy your Scouting you can by joining Scout Network. As a Scout Network Member you get to take part in all the usual great Scouting activities such as kayaking, climbing, orienteering... as well as work on projects helping to support you local community.
You can also, if you choose, get more closely involved in working with the younger Sections and providing them with the same kind of opportunities you've had through Scouting.
To find out more about the Scout Network see https://www.scouts.org.uk/network.