The most important thing to understand here is that the person who emerges from their early to mid-twenties is pretty much the person you're going to have for life. Good hiring is important because the only things you're going to be able to teach someone involve knowledge and skills, not motivation or attitude or cultural fit.
Beyond that, the best clue you've got to how someone will perform on the job, and how they will learn, is what they've done in the past. You can often get specific past performance to review with older workers, with younger ones it can be more of a crapshoot. Still, there are some things you can look for.
Look at the important behaviors you have for your job. Has the candidate done similar kinds of things before? Extroverts don't become introverts in mid-life. Detail people don't suddenly become big picture people. We expect mangers to help their people grow. Has your candidate done that? Pharma sales people need to master a lot of technical material. Has your candidate done that?
Take special note of the uncomfortable parts of the job. For management jobs for example you must engage in "controlled confrontation" with subordinates on a daily basis. Has the candidate demonstrated that? A tech support person who works the help desk should have demonstrated the ability to work with people on the phone. One who goes on the road should have demonstrated self-starting and organizing behaviors.
If they've shown they can handle the important and uncomfortable behaviors, or behaviors very much like them, they can probably learn any technical material you throw at them.
Please note something in what I've said above. I've constantly used words like "demonstrated" and "behavior." Candidates will use courses and classes as qualifications. They're helpful in assessing knowledge in some cases, but the best guide to future behavior is past behavior.