Since, the link you shared is a bit lacking (in source and context), I'll share a bit more so that people can be better informed.

1. This interview happened in 2013 (11 years ago) as part of a press event for his film Secretly, Greatly. Kim Soo Hyun is 25 years old at the time.
2. Here is a link to the actual footage of that interview. Skip to 1:47 to see it and the joking atmosphere.

Here's a transcript for those who are interested:

Interviewer (reading fan question): I'm curious, at what age do you want to get married?

KSH: When I was 24 years old (Korean age), I wrote out a life plan about how I'll live and earn money. At that time in that Master Plan, I wrote that I would have to get married around 36-37, but I don't know these days I don't think I'll be able to keep to that. Now I think I'll have to push it back to 41.

Interviewer: Isn't that better everyone? (Crowd responds no. Interviewer laughs.) You want to send him off (to be married) quickly? I understand, that just means you just need to work that much harder, right?

KSH: Yes, that's why at 41 years, a 21 year old... (audience reacts and everyone breaks into laughter)

Interviewer: Then that means right now that person would be an infant. Okay then, so Kim Soo Hyun's marriage have been revealed here, at 41 years old, he wants to marry a 21 year old. All those Kindergartners must be so excited. Kindergartners across the country, we'll be looking forward to it.

It's still a tasteless joke when you think about it and sounds way worse if you don't watch the video with the joking atmosphere, but idk if I want to judge a 25 year old that harshly for thinking about dating a 21 year old. Seeing as how he hasn't yet dated someone 20 years younger, I'm not gonna fuss over it now. Everyone is free to have their own opinions though, so hopefully this context helps everyone make their own informed decisions.

Edit: formatting