In Sweden the age of consent for sexual activity is fifteen years. Sweden ranked the 9th happiest nation in 2016. In 30 states of the USA the age of consent is 16 years.
In my opinion the debate should be about "Did Joseph Smith manipulate?", not about age because marrying teens was not illegal in the 19th century.
Today, no one sees anything wrong with a 30 year old man marrying an 18 year old, despite the fact that many 18 year olds are still not mature enough. The problem is the manipulation, not a 12 year difference, 30 - 18 = 12