Dating people who are younger then your children?

So I want honest opinions on this. This happens quite a bit and I try not to judge people who date people with large age gaps. But somehow it’s weirder to me when the person has children that are older then the person they are dating. My SO dad it’s 45 my SO is 23 and his sister is 25. His dad has a new girlfriend she is 21. This is so weird to us and we can’t even take her seriously as a person he is dating.

I just couldn’t imagine dating someone who is younger or even the same age as your child. So what’s your opinion about people who date people who are younger then their own children?