If they have already seen people that by far exceed the standard, why shouldn't they start putting them on the shortlist? By no means are they completely filling it up with the early applications, but they have been doing recruitment for a bloody long time, and know exactly what they and the airlines are looking for. Hence they are able to use this experience to identify candidates that they know are the right ones. There will be people from the first few days that only get shortlisted once everyone has been seen. I was on the second to last selection day last year (from the first round), and got shortlisted, along with most of the people that made it to the interview stage. And bear in mind, they only invited 120 people to do selection at Dibden, and shortlisted 50. so if they see a few people they like early on, they can offer them a space, with more than enough places remaining. They know how tough selection is, and that not all 120 people will pass (6/12 on my day). So shortlisting people early is a complete non-issue. Caveat being i don't know the numbers this time around, but it will be similar. I don't think its a case of just trying to fill spaces, as they get so many applications that will never ever be a problem.
My advice would be to please just concentrate on yourself. If you decide to concentrate on trying to 'play the game', get too focused on age and how many other people you are contending with, your performance will slip. Go and try your best, and if you have what they are looking for, you will go all the way. If you go and try your best and still don't make it, then yes its disappointing, but use it as an experience, learn from it to be even better the next time (took me 3 goes). Or you can sit there and be bitter, saying you were too old or the recruitment process was unfair. Because after all being bitter is a very valuable quality for pilots.