Concessions can be upgraded OR shared but only with a MyAlbion+ member who meets the concession criteria. So an U18 can shared with an U18 or an 65+ can be shared with a 65+ member. If the MyAlbion+ member doesn't meet the concession criteria then an upgrade is required.
Anyone looking to get MyAlbion+ membership (which yes does need to renewed each season) there is a discount code which is valid until 23:59 tonight (30 June):
STFRIEND
Which will give you adult membership for £28 and U18s membership for £18.