The 27-year-old, who previously starred in The Inbetweeners, is currently appearing in Channel 4's comedy Fresh Meat.
The show focuses on a group of students living in a house together and Thomas has now told The Sun that he is often asked to play someone younger than himself.
"I think I stopped maturing quite a long time ago, physically at least," he joked. "I've never been cast as someone who has sex."
Thomas added: "This face screams 'virgin', apparently - which is good to know."
Fresh Meat's writers Sam Bain and Jesse Armstrong recently insisted that they do not "censor" themselves when they are writing the show.
Fresh Meat airs on Wednesday at 10pm on Channel 4.
> Jack Whitehall Fresh Meat interview: 'I play a posh idiot'