I'm sure this will bring out more great candidates for the job, however if you're about to hit 40 when you enlist, the Air Force may not end up being able to use you for as long, as there's a much higher chance of injury, early onset health issues, etc. But from a strictly "quality" perspective, it opens the door for many people who would have been great candidates when they were younger but just hadn't considered it an option at that point. As a young person myself, I know that a lot of us are not yet ready to commit to a path. And some of us have committed to a path that won't end up being the right one, because we don't have the experience to foresee things like glass ceilings, salaries that won't support our lifestyles ten years down the road, etc.
A lot of people get great experience just out of school and by the time they realize that the military is good option they have already aged out of the process. In today's military, it's not all about being in top shape and being able to run forever. These days, we need intelligent computer experts, drone pilots, and logistics managers. There's no reason these jobs can't be done by older airmen.
Yes, raising the age limit to enlist from 27 to 39 will likely help with the quality of people who enlist. Older people tend to be more mature, have life experience and be focuses on a career plan. However, with age comes a natural slowing and physical decline. These factors could also lower the quality of candidates.
Yes, this will help with the quality of people who enlist. Older people who enlist will probably be wiser and have more knowledge of the Air Force and how to proceed in it. This is a good idea as the Air Force will rule out people who are a little more childish.