How does a 20-something find a white collar job with lots of 20-somethings? Statistics/finance/accounting/applied-math field.
Apply to them and see who interviews you. If they are on the younger side chances are the workplace will be predominantly younger people. If the interview is on-premises it should also help you get a feel for it.
Smaller, newer companies also tend to have younger employees in general compared to older companies or larger ones with people that have been working there for decades or hierarchy so bloated people with decades of experience need to handle parts of it.
How does a 20-something find a white collar job with lots of 20-somethings?
Many of the best jobs are found through your personal or professional network of friends. If you have friends who are in their 20s and working in white collar jobs, I'd start there.
Another option is to look at company websites in their careers section. If the language talks about the kinds of things 20 somethings would like it's a reasonable guess that they are trying to attract candidates like you. Also, if they show pictures of folks around your age, that's a good sign.
If you are recent graduate, you should consult your university's placement department. Often they have insight into companies hiring new graduates.
Finally, consider working through an agency. They have insight into which companies are hiring, and what kind of culture you could expect.