Since "shallow" may be interpreted in many ways, I am referring her to people who put value physical appearance to a higher importance than other traits (character, maturity, personality ...etc).

Men find women between ages 20-24 the most physically attractive regardless of their own age. Women, on the other hand, tend to find men around their age (a bit older in their 20; a bit younger in their 30) more physically attractive.

https://www.businessinsider.com/dataclysm-shows-men-are-attracted-to-women-in-their-20s-2014-10?r=DE&IR=T

A man in his 30' dating mostly women in their early 20 might/is considered shallow as he puts physical appearance above other traits (character , personality , maturity; all which take GENERALLY more time to develop)

Women mature faster than men. There are several studies on this topic. Example:

https://www.psychologytoday.com/us/blog/the-athletes-way/201312/scientists-identify-why-girls-often-mature-faster-boys

Hence a woman to date on her own maturity level would probably need to date older. This means that women dating in their own range put physical appearance over other traits (maturity, personality, economics; all which take GENERALLY more time to develop)

Are the women not dating older men shallow?