'Twerp' & 'twit' seemed slow migrating to Oz if they were current there in 60s, as stated above. I remember them here in England from 1940 — I have a vivid memory of having been denounced as both by team-mates for an inadvertent handball during a game of football when I was 8 years old in Worthing (also, simultaneously, 'flop').

I can date this so precisely because it was during my first couple of days at Goring Hall School. Funny the discrete & trivial experiences that one's memory randomly stores.