Public recruitment typically happens at university, which is dominated by young people.
Older people who show up at the university job fair or get “encouraged” by faculty with connections get recruited just the same.
In fact, they’re not recruiting 20 year olds, they’re more likely to recruit people in their late 20s or even 30s who have some real life experience and travel under their belts.
Caveat: apparently service in the Peace Corps and certain other NGOs will disqualify you for Officer duty. At least that’s the claim, backed up by several ex-officers.