ML acts as a normal 19 year old - full of emotions and figuring them out. MC is very mature, and I was concerned for the age gap, but MC is more like a mature child than an actual adult. Even though he is 69 years old, he lived the 50 years in constant survival and agony to actually grow. He became emotionally mature, yes, but later in the novel/manhwa you can see that he is just a child the more he spends time with ML. Mentally, he stayed as a 19 year old.
As how he sees younger people - in the novel he describes that he was always out of touch with his peers (because he was getting ready to go to the future from when he was 18) even before he went to the future and is just patient with them.
50 years age gap, and not just usual mental age gap, but war veteran who had lived in harshest timeline. Tbh, if I were MC, I won't remember friends from 50 years ago. Not even first love. Life goes on, in 50 years, many things had happened.
I wonder if author can really depict how older person actually think about younger people.