Which is a bit odd because Letitia Wright is like 24. Not only is it "Dawson's Creek casting," but her being a teenager isn't really mentioned in the film, just that she's "young." So you could have had Shuri be in her early 20's and nothing really changes (since most of the other main character still seem older than she is regardless).
But yeah, Shuri's age makes any potential romance with Bucky (who even excluding the cry-sleep stuff is still probably in his 30's still) extremely unlikely. However, them becoming good friends is something that might be cool to see.