At my current company we ask even juniors to get involved. Interviews are in pairs, with at least one senior person in that pair, so we can make sure each interview is as fair as possible. IMO everyone can have a valid view, and since we could well be looking for their next team mate I'd prefer to know that it's not just my opinion that gets heard. It's been working pretty well so far, and given people more experience in hiring that I hope helps them later in their careers.