From my memory the Blondie is right. It was 19 to 35 for bethel service. However, being a pioneer was the extra credit that would get you accepted but not a requirement. Non-pioneer brothers have been accepted to bethel. Obviously, it's the exception not the norm.
Also, perhaps by inviting 18 year olds to the admittance meeting, which is a stupid waste of time meeting, they can get them primed to apply when they're 19. Although I thought bethel wasn't in need of many new ones, since they've been downsizing.