Age-Gap Algebra

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Numbers don't lie!

Jenny: I'm 8. That means that when I'm 18, you'll be 28. And when I'm 28, you'll be 38. [..] And when I'm 38, you'll only be 48. That's not much difference at all.
Edward: Sure seems like a lot now though, huh?
(twenty years later)
Edward: Your name's different. Did you get married?
Jenny: Yeah. I was 18, he was 28. Turns out it was a big difference.

One common criterion for compatibility between prospective marriage or romantic partners is the difference between their ages. This trope is when a specific formula or calculation is proposed to determine either the "ideal" age gap, or the maximum allowable age gap before it becomes "creepy".

Traditionally, a man was expected to be established in his career and lifestyle before marriage, whereas a woman usually transferred directly from her father's household to her husband's within a couple years of reaching adulthood, so it was considered ideal for the man to be at least somewhat older. As Society Marched On, this became less and less the case, and in modern works a very small age gap is often considered ideal, with a maximum allowable (as opposed to "ideal") age gap proposed, and often applied equally in either direction (older man vs. older woman).

One commonly-applied formula is the "half your age plus seven" rule, in which the older partner's age is divided by two and then increased by seven to reach either the ideal or minimum allowable age for a romantic partner. This has the advantage of allowing for a larger age gap the older the partners get; the four-year age gap between a 22-year-old and an 18-year-old is significant (and just barely allowed by this rule), but the same age gap between an 86-year-old and a 90-year-old isn't worth comment. The rule would theoretically create a paradox for people under 14, because the younger partner would have to be older than the older partner, but 14 is about the youngest you can be and make anything even remotely resembling adultish decisions respecting relationships anyway, so this rule works out rather well. In its earliest appearances, this rule is often cited as having French origins, although this citation always seems to appear in English-language (British or American) sources, leaving its true origin mysterious.

Another, somewhat vaguer metric often cited is declaring an age gap creepy when a partner is "old enough to be your mother/father," carrying with it as it does implications of symbolic Parental Incest or Wife Husbandry. This would place the maximum allowable age gap at a static fifteen to twenty years or so, depending on if the speaker is referring to the physical ability to have children or the socially acceptable age.

Compare May–December Romance, Lolicon, Likes Older Women, Likes Older Men, and Wife Husbandry.

Examples in fiction:

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    Film 
  • Big Fish: While in the town of Spectre, Edward befriends a young girl named Jenny, who develops a crush on him. He's eighteen at the time and she's eight, but that doesn't put her off, as she says once she's eighteen he'll be twenty-eight and it won't be such a big difference. Twenty years later he meets her again and finds she's divorced from a previous husband, who was also ten years older than her. According to her, it turned out to be a big difference. As for she and Edward, they don't get together, because he's already married to his One True Love.

    Literature 
  • In The Autobiography Of Malcolm X, Malcolm X cites the half-your-age-plus-seven rule as having factored into his decision to propose to his future wife.
  • One of the first recorded instances of the half-your-age-plus-seven rule is in French-born author and journalist Max O'Rell's 1901 advice book Her Royal Highness Woman. It may predate O'Rell, as he presents it not as his own invention but as a very good piece of advice that he "heard the other day." A reading of the original text reveals some of the Values Dissonance inherent in the idea that a man "should" be older:
    Never marry a woman richer than you, or one taller than you, or one older than you. Be always gently superior to your wife in fortune, in size, and in age, so that in every possible way she may appeal to you for help or protection, either through your purse, your strength, or your experience of life. Marry her at an age that will always enable you to play with her all the different characteristic parts of a husband, a chum, a lover, an adviser, a protector, and just a tiny suspicion of a father.
  • In the 1903 novel The Little Shepherd of Kingdom Come, Miss Jennie cites the half-your-age-plus-seven rule as a French saying, possibly referencing Max O'Rell.

     Live-Action TV 
  • In Parks and Recreation Andy (29) asks Tom how young is too young to date. Tom quotes the half-your-age-plus-seven rule, which in Andy's case would be 21 and a half, causing Andy some consternation when he realizes this technically puts the 21-year-old April in the "too young" range. He gets over it. It helps the audience that the age gap between the actors is only five years instead of eight (and that Andy's mental age at best is around 21).
  • In Friends, Joey apparently gets his algebra mixed up, making the age gap sounds even worse than it is:
    Joey: Yeah, come on, think about it. You're 18, okay, she's 44. When you're 36, she's gonna be 88.
    Frank: What, you don't think I know that?

    Theater 
  • In The Moon Is Blue, Patty is 21 years old, and finds out from Don that he's 28 years old. (In the film version, she's 22 and he's 30.) She's quite delighted with her calculations:
    Patty: Isn't that amazing? It just works out.
    Don: What does?
    Patty: Haven't you ever heard that the girl is supposed to be half the man's age, plus seven?
    Don: What girl? What man?
    Patty: Never mind. Beat it.
  • In Princess Ida, Prince Hilarion reflects in a song that, at the time of his Arranged Marriage, he was twice Ida's age, and that "augurs ill for married life." Still, he was only two then, and she has gained on him in the twenty years that have passed.
  • Abbott and Costello had a routine where Costello claimed he was 25 and had fallen in love with a five-year-old. Abbott suggests if Costello wait five years, he'll be three times her age instead of five times. Abbott then suggests if Costello waits ten years more, he'll only be twice her age. Costello asks Abbott how long until he and she are the same age.

     Visual Novels 
  • In Hustle Cat, the Big Bad specifically references this trope if you're on Graves' route, as Graves is twenty years older than nineteen-year-old Avery.

     Webcomics 
  • xkcd #314 shows the "Standard Creepiness Rule" listed as "Don't date under (age/2 + 7)", providing a minimum age if not an ideal age.

     Web Original 
  • The May–December Romance trope on This Very Wiki specifies an age gap of at least thirty years before examples qualify.
  • In xkcd's spinoff WhatIf, a writer asks what would happen if each person had one soul-mate and could be happy with no one else. Monroe ends up using a age limit much more strict than 1/2 + 7, arguing it would be creepy if a couple met as children (30 & 40 isn't so bad, 5 & 15 is a problem).
http://tvtropes.org/pmwiki/pmwiki.php/Main/AgeGapAlgebra