"Company X isn't looking for anyone seasoned"
"Company Y is open to seasoned individuals"
Companies would also look for people who were a certain number of years of out school. Like, "5 to 10 years experience post-degree." Sure, that could technically mean a 45 year old who went back to school at 35 is qualified for the position, but there is a 0% chance the company would hire that person.
The other recruiters would talk about these things pretty explicitly outside of emails. There were a lot of interesting justifications:
Older people are harder to train
Older people are difficult employees because they are stuck in their ways/used to getting their way
Older people expect more benefits/vacation/pay
Older people use more sick time
Older people don't keep up with developments in the field
Discrimination wasn't limited to age. The recruiters were just as happy to help their clients discriminate on race, or gender, or familial status. Really anything.
In my experience, age was the only protected class that recruiters would try to suss out how the hiring managers felt about while they were trying to determine the ideal candidate. It seemed to me that the other prejudices would only come to light after the recruiter had a relationship with the hiring manager and had sent them a number of candidates in the past. There were still a lot of euphemisms though.
This was just my experience though, and I was working for a small firm. I have no idea what the rest of the field is like at all.