I think that Mary was actually only 13 when she got engaged!
I hate when shows have a character do an about face just to create drama, which I think is why they had Charles insist on Laura waiting until she was eighteen to get married. "I don't think a girl should get married until she's eighteen." But Mary was only sixteen. Laura should have told him to make up his mind.
Yeah, "Manly" and Laura's age difference was skeevy at the beginning. Especially in those episodes where she was wearing her hair down and loose. IMO it made her look even younger. And he kept acting like he couldn't wait to jump her bones.