Berghaus Antaeus Rucksack for sale as seen below. Main body is 60L with side compartments adding 10L. Also has a sleeping bag storage bit at the bottom, or for whatever you want to put in it.
This is a womens bag, but would suit a smallish framed teenager or man as well. I'm a guy and I bought it at the age of 15 for my bronze D of E award. It's a great bag and it's given me many uses, but still in great condition. Absolutely bomb proof. Also has the Berghaus biofit back system, so it can be adjusted to any bag length really.
Only asking for £30 + £5 posted, or collection from Oxfordshire (PM me for an address).