If the results of a recent survey are anything to go by, this rule has been well and truly thrown in the garbage.
According to research from the world's largest dating app Badoo, many millennials are trying to find a partner who is significantly older than them.
A sample of 10,500 people on the Badoo database revealed that 26% of 18-24 year olds would date someone over the age of 35. Nearly a third of women have dated someone 10 years older, and 9% of men would date someone 20 years older than them.
Abbie Moujaes, Badoo's in-house dating expert, told Business Insider that millennials may be more accepting of age gap relationships because there are so many famous couples following the trend. For example, Emmanuel Macron (40) and his wife Brigitte Macron (65), and George Clooney (56) and Amal Clooney (40).
"Millennials are looking to date someone who is older; as with age comes maturity and also the connotation of having your life together," Moujaes said. "Dating someone older can be seen as a fast-track route into leading a stable life, which for a lot of millennials is enticing."
In other words, while millennials are facing the prospect of a life of never owning their own homes and earning less money than the previous generation, they may be turning to older partners because they're more likely to be in a better financial place.
There is little scientific evidence that women who go for older men have broken attachment styles — known colloquially as "daddy issues." (Just in case that's what you were thinking.) For example, one study from 2016, published in the journal Evolutionary Behavioral Sciences, found that women in couples with small and large age gaps were similar in both attachment style and relationship satisfaction.
Another speculative theory is that millennials and older generations simply have more in common than they used to. Thanks to on-demand TV and pretty much anything being available on the internet, younger generations have been able to grow up with the same television shows, films, and music older people did. Millennials were arguably the first generation to be able to do this significantly, and choose older partners as a direct result of it.
Having children is also a factor. In 2011, The Center for Work-Life Policy published a study that showed 43% of women and 32% of men in generation X, the generation before millennials, were putting children on hold, or deciding not to have them. Millennials are also trending towards having fewer children, with birth rates dropping around 15% between 2007 and 2012.
So perhaps millennial women who aren't keen on having children early, or ever, prefer to date older men who might be more on the same page rather than younger men who might rush them into starting a family.
Ultimately, the survey does suggest many millennials are looking for a relationship with older people, and the reasons for that are completely speculative. Some people might just like the idea of dating someone with more experience — and dating sites are a really easy way to make that happen.