I have seen the real Verlaine. I knew about them prior to seeing this film.
Genetics and such can play a part but a hard life and/or hard living doesn't help.
People also generally aged faster then than now.

I was about to say the same thing. Alcohol does play a role on someone losing their youth but if you saw the real Paul you would see that he did look older than his age simply because he was balding in his late 20's and had a beard.

10 years is quite a bit when someone is a teen. People aged more quickly during that time and genetics and lifestyle, the absinthe and such, often plays a part.

Yes, that could be true. Perhaps if it had been a younger man playing Verlaine, we would've felt more sympathy and affection for their relationship, and the impact of their twisted minds would've been softened. Maybe they wanted the primary focus to be on the disturbing personalities of the poets, not on how they looked together. Leo already passes for a teen easily though, so was it really necessary? I think a younger actor could be equally capable of portraying the psychotic, abusive Verlaine. I also felt the two lacked chemistry, but I haven't watched this in so long so my memory is hazy.
But from a realistic viewpoint, the role is pretty provocative and perhaps controversial (nudity, gay scenes, etc). Maybe they didn't have a large pool of potential Verlaines to begin with, and Thewlis was the best fit. Not sure, really.

Maybe it was done to make the realtionship look even more taboo?

That's what I was thinking! Even though David Thewlis did a fine job in his role, I just wish they had picked an actor who appeared a bit younger? Nonetheless, Thewlis did well.