People love using idiotic "rules" like that as an excuse to be judgemental assholes.

It would be much better if we taught people to think for themselves.

Most women most often date men a few years older than them. Mary Tyler Moore married a man 20 years younger. Rather than do math to see if she fits into some baseless "rule", we could just wish her a very happy life because there's not a shred of evidence that age-gaps are harmful in any way. We really need to STFU and mind our own business :-)