Lilmod ulilamed-I have a friend whose husband was an older single boy until he married her. He smoked until he got married (she didn’t find out until right before they got engaged or she probably would never have ended up going out with him or marrying him). When he mentioned that he used to smoke, I was shocked because he is a really serious Ben Torah and very straight and shtark. He explained that as an older single boy (maybe even just as a bochur even w/o the older part), he needed an outlet. He needed to do something that felt “rebellious” but was not actually assur (at least not in the way that movies, hanging out with girls, etc. are), so that’s why he smoked. My impression is that is why a lot of bochurim smoke.
There are a lot of choshuve serious bnei Torah that Learn all day that smoke.
It’s not halachikly wrong