I wonder why he used the word 'pure' instead of 'innocent'. I think Hashi is really exagerating his love and respect for Sasuke's clan there a bit. It almost looked like he tried to protect him from any harm and from someone hurting his feelings. That made him the total opposite of Tobirama who pretty much wanted to insult Sasuke from the beginning on.
I would never refer to a teenager as being pure or innocent. That's more a term for a baby or a little child. Weird choice of words.