Here's another example describing Snape's handwriting at age 15:
Harry moved around behind Snape and read the heading of the examination paper: DEFENCE AGAINST THE DARK ARTS – ORDINARY WIZARDING LEVEL.
So Snape had to be fifteen or sixteen, around Harry’s own age. His hand was flying across the parchment; he had written at least a foot more than his closest neighbours, and yet his writing was minuscule and cramped.
Order of the Phoenix - pages 564-565 - Bloomsbury - chapter twenty-eight, Snape's Worst Memory
A few things:
When it comes to a canonical characterization (I believe as you seem to), Hermione ostensibly should have noticed, if Snape's handwriting remained the same from his teen years to adulthood -- this is extremely unlikely. There is nothing in canon that directly addresses this.
J.K. Rowling needed Hermione to not notice because JKR needed us not to know who the Half-Blood Prince is until the story is wrapping up. Harry needed to be the one who discovers the truth to the secret of the Half-Blood Prince.
As DVK and alexwlchan both mention, handwriting can change significantly between the teen years and adulthood. Some similarities exist (see out-of-universe example below) but the finished products could easily be mistaken for separate writers if one isn't looking with a super keen eye.