Who was considered an adult changed over time as our society changed. When there were no schools, most labor was physical, etc. then puberty was often seen as the natural line between children and adults because a 14-15 year old could have all the attributes and adult had: job, income, status, etc. As we moved to a society where school was mandatory till 18 (and now nearly till 22), our attitudes about when people become adults also changed. Now we are at a point where a 20-30 year old adult being with an 18 year old is seen as taboo, even though all are adults, this is largely because as a society we have moved to a point where childhood has extended from 18 now until one is secure in career and finances (mid 20s).

I'd still maintain that marrying with large age gaps isn't child abuse, and when you claim child abuse is condoned by the Mormons you know what people assume vs what it actually is. You are just trying to be inflammatory.