Harvard Adcom Brit Dewey stated something to this effect years ago. When a reporter pointed out that discriminating against older applicants because of their age was illegal, she got really quiet.
I do also recall adcoms from Wharton and UCLA stating that they are interested in "early career candidates". They can't discriminate based on age, but they can adjust their admissions policies to favor people with less experience, essentially those that are younger.
Want some evidence? How about a link to the actually policy on the UCLA Anderson website:
https://www.anderson.ucla.edu/x9493.xml
Still not convinced because "a glance at MBA program websites shows class profiles that seem skewed to individuals with 4-6 years or more of work experience."? Funny, this exact same question was addressed on the official Wharton adcom blog. Check out this link for a discussion on early career candidates at Wharton:
https://adcomblog.wharton.upenn.edu/admi ... ns__2.html
The simple summary is that many schools are actively encouraging early career candidates. Harvard and Stanford have been doing it the longest (perhaps 6-8 years) and have markedly lower averages for age and work experience. Wharton, UCLA and others have adopted the policy more recently, in the last several years. The Wharton Adcom did explicitly state in a chat last year that they were increasing the number of early career they were admitting. What written proof? Try doing a search. I can't do all the work for you.