Experience points for solved low level sidequests?
If a quest is more than 5 levels below you (turning the quest level color from green to grey in your journal), then you get a tiny xp reward. If you want the maximum xp reward, the quest should be completed while its still "green" for you. The actual numbers depend on the quest itself. 5 xp is not the minimum for all "grey" quests, its just the amount that quest pays out when it is "grey". Some "grey" quests may pay out a little more than that, and some may pay out none at all.