The 19-year-old started alongside Reds team-mate Marouane Fellaini as manager Marc Wilmots made seven changes to his side, who had already qualified for the knockout stages but made it three wins from three courtesy of Jan Vertonghen’s late goal.
Norman Whiteside, who was 17 when he played for Northern Ireland in 1982, is the only Red to have taken part in the tournament at a younger age than Januzaj.
While Fellaini was his usual rumbustious self in central midfield in Sao Paulo, Januzaj saw little of the ball on the right in the first half, though he was neat and tidy in possession.
In a tight opening 45 minutes, Dries Mertens’ glaring miss from six yards and Thibaut Courtois’ superb save from Ki Sung-yeung’s low drive were the only real talking points, until Steven Defour was shown a straight red card for a tackle on Kim Shin-wook on the stroke of half-time.
Early in the second half, Fellaini did enough to force substitute Lee Keun-ho to head over a good chance from close range as Korea sought to take advantage of their extra man.
The rangy Reds midfielder had a penalty appeal waved away after appearing to be brought down by Hong Jeong-ho as he charged into the box, before almost getting on the end of Januzaj’s cross minutes later.