I love this character but something's amiss. He appears to be about 12 years old in the movie, a similar age to Mary (Margaret O'Brien) and Colin (Dean Stockwell). Yet according to IMDb, Roper was born in 1929 and therefore would have been about 20 when the movie was made. Is that plausible?
It's possible that he was small for his age. It's been common practice in more recent years for actors in their 20s to play teenagers (due to child labor laws and legal issues). And maybe they weren't able to find younger performers who would have been convincing in the part. The fact that Brian Roper was a native of Yorkshire no doubt had bearing on why he was cast.
Dean Stockwell was about 13 during filming, playing a 10-year-old; Margaret O'Brien, also portraying a 10-year-old child, was 12 years old. Natalie Wood also tended to play characters younger than her actual age in her years as a child actor.
Yes, and in Dear Ruth/Wife/Brat (1947/49/51) Mona Freeman played an approximately 18-year-old brat when she was 21/23/25. But the physiological difference between 20 and 12 is significantly greater than that between 25 and 18 or between 12 and 10. I'm going to watch The Secret Garden again and see if I can convince myself that Dickon is played by a 20-year-old.
Brian Roper later reprised his role in the brief television series based on the film in 1952. It would be interesting to compare it to the film, but unfortunately, all of the episodes appear to be lost.
It would be nice if there was more info about him too.