https://link.springer.com/content/pdf/10.1007/s12144-018-9895-6.pdf

Age-gap relationships were harshly judged by people and seen as inequitable. They believed that the older person would receive more benefits at the expense of the younger person. Nonetheless, the participants rated the older man dating the younger woman more harshly than the older woman and younger man. Both however, were rated much more harshly compared to age similar couples. There were 99 participants ages 19 to 63 years old (mean age=29.49, SD=7.23) (62 men, 35 women, and 2 people who did not pick a gender) recruited from Amazon's MTurk.

As seen in Fig. 1, people expressed greater prejudice towards young men’s relationships when they partnered with older (M = 55.45, SD = 32.60), rather than younger women (M = 81.95, SD = 20.17), t(97) = −7.45, p < .001, d = .97.1 Similarly, people expressed greater prejudice towards older men’s relationships when they partnered with younger (M = 50.70, SD = 33.04), rather than older women (M = 80.08, SD = 22.45), t(97) = −8.34, p < .001, d = 1.04. Planned contrasts revealed that, in general, age-gap couples (M = 52.84, SD = 31.36) were liked significantly less than age-matched couples (M = 80.70, SD = 19.19), t(97) = −8.61, p < .001, d = 1.07.

Older men dating younger women were rated 50.7 (SD=33.04) but older women dating younger men were rated 55.45 (SD=32.6).

https://preview.redd.it/ac0s5osnk6ha1.png?width=1176&format=png&auto=webp&v=enabled&s=f1aee1d18eba48d55e2de3df1bd0873814a1ccdc

Then participants rated how they perceived inequity in relationships based on age-gender dynamics. "Participants inferred the extent to which each relationship was inequitable using a 7- point scale, ranging from 1 (He is getting a much better deal than she is) to 7 (She is getting a much better deal than he is). The midpoint of the scale was labeled 4 (They are both getting the same, equal deal)." They found that the more a person perceived inequity in an age gap relationship, the more harshly they judged an age gap relationship, but the more they perceived inequity in an age-match relationship did not increase their negative perception of an age-match relationship. Still though they perceived more inequity with men dating younger women rather than older women dating younger men.

https://preview.redd.it/f6vwh0zbl6ha1.png?width=1124&format=png&auto=webp&v=enabled&s=8d3886351a3b4f1543211b3172bc317830ab26a8

And age gap couples were judged much more harshly than mixed-socioeconomic status couples (i.e.: rich dates poor), mixed-weight (i.e.: fat person and thin person together) couples and interracial couples.

https://preview.redd.it/9wj1pm5al6ha1.png?width=1087&format=png&auto=webp&v=enabled&s=066dd67b1ba9fca64fe7ba4defaa2e76d06e7327

Some older studies do show that women dating younger men is judged more harshly, but those were studies back in the day. It seems the double standard has switched. Men dating younger women is more taboo now, and cougars are more normalized now, probably due to the MILF porn category, which was set off by American Pie.