The reason Jang Sung-kyu was surprised upon meeting a man and a woman at the Han River

What do the numbers 79 for men and 42 for women mean?

The man is 79 years old, and the woman is not 42 years old.

The man was born in 1979, the woman in 1942. What could it be?

There may be someone who questions this.

Try to guess it.

I will leave the answer in the comments.