I don't understand how an age gap of 12 years between two consenting adult men will affect their performance. You're watching the show for the characters and the storyline, not to ship them in real life.
How does someone change their personality like that in a span of a few years?
But you have to realize that Wen Ning's "personality" is the direct product of his childhood trauma. People don't just "get over" trauma that easily. I understand maturing, but his personality took a whole 180° turn.