Basic Trope: Lolicon, where the young girl is legally married to the older man.
Straight: Alice is 12, and she's married off to Bob, who is 50.
Alice hasn't even reached puberty yet, and she's already being married to Bob.
Alice is 12 and Bob is 80.
A 100-year-old Bob gets married to Alice while she is still a fetus in her mother's womb. (No word on how Bob knows the fetus is a girl.)
Downplayed: Alice is 15 and married to a 27-year-old man.
Alice and Bob come from a time and/or place where young girls are married off as soon as they reached puberty, to older men who could provide for them (and pay a nice bride price to the girls' families).
It's illegal in the setting to marry before age 18.
Conversed: "Can you imagine living in a place like that, where a little girl could be married off to a middle-aged man?"
Deconstructed: This can lead to problems; Alice's reproductive system may not be ready to handle Bob's full-sized penis yet (which can lead to injury and infertility), and younger mothers have a hard time giving birth, which could result in Death by Childbirth. Not to mention, Alice may not be given a say in whom she marries (especially if this is a cultural thing).
Reconstructed: Alice is betrothed to Bob, but they will wait until she's old enough before actually getting married or consummating the marriage. And either one of them can break off the engagement at any time.