My grandma was 14 when she first married and had her first kid. Not Mormons, but it use to be very common and I didn't see anyone getting hurt. I don't advocate anyone getting married or having children till they are 30. But thats just me. I have no right to force my opinions on anyone else. By the way grandma had 3 marraiges and outlived all her husbands to a ripe old age of 96.
The only criteria there should be in these cases is somebody being hurt. If not leave them alone and pay more attention to your own life.