They're not Mormon, they're Evangelical Christians.
Taylor got married at 19 because he knocked a girl up. They got married right after she graduated from high school, in June of 02. It's said he dated her for two years prior, but there are other stories.
Zac got married at 20, but he dated his girlfriend for 5 years beforehand and proposed like a year or something before the wedding, and they got married right after she graduated from college, in June of 06. She was 22.
Isaac was 25 when he got married, iirc. His wife, Nikki, was 22, and they got married at this huge unmatched ceremony (lol seriously, the colors? insane) in September of 06.