Here we see Frank Sheehan during World War II, and in the below picture, we jump forward a couple decades, presumably sometime in the 1950s or early 1960s when he’s still relatively early into his career as an assassin.
While I wouldn’t go so far as to say it’s the same level of quality we’ve seen in recent Marvel Cinematic Universe movies, this is still a solid example of the lengths Martin Scorsese and his team went to make Robert De Niro look younger in The Irishman.