DD1, who was young in her year(june birthday) started school in the January aged 4.5(so not a full year in Reception) had picture and library books from the start and proper books after about a term. She was reading fluently from age 5 and a half and onto things like The Worst Witch and Enid Blyton from a few months into year 1. She acheived level 3 reading in KS1 SATs
DD2 (September birthday) was into reading books after two weeks but was not IMO ready for them, became very frustrated and didn't really start to read fluently until the middle of year 2. She also achieved level 3 in reading at the end of year 2.
They all make progress at different rates.
Don't be too keen to push them into to formal reading before they are ready.