The reader page at http://clojure.org/reader states that symbols (and keywords) cannot start with a number and the regex used in LispReader (and EdnReader) also has this intention.
CLJ-1252 addressed this by fixing the broken reader regex to match the spec. However, that broke some existing code so we rolled back the change. There is still a disconnect here and this ticket serves to decide what to do instead.
I presume that we are effectively deciding that keywords like :5 are ok to read. If so, we should alter the regex to more accurately capture that intent - right now it allows these purely by accident due to backtracking. A secondary question is whether the Clojure and EDN reader spec should also explicitly allow these as valid. My preference would be to have the reader and the spec match, so I would lobby to loosen the reader spec.