Possibly because it is handholding them through the process. A more mature candidate would not need such direction, and might see it as not a good match for them.

Also, not to pick, but the owl character that might seem like a good idea to younger candidates, that might respect it for the good design and soft psychological impact, might scare off the mature employees that are there to get the job done and are there to engineer solutions.

Despite these things, I think it looks professional, and the fact is that if you're a startup, you're most likely looking for a younger candidate, because they are cheaper, can be more motivated, and are definitely easier to mold. And yes, if you discriminate, that is illegal in the U.S., at least.

However, at some point, you need to hire more experienced candidates, and this process will need to change to reflect that. I'm a more mature person myself, and would not seek to fill a development team with juniors. I've seen what that can do. It's fine to have interns and juniors, but that shouldn't be the primary talent pool, or you'll end up with serious design problems and lots of poor code. But that can happen with more experienced candidates also- that is why you want good, more experienced candidates to create an environment that fosters growth of incredibly smart juniors.