My senior 3 has 10 kids- 3 of them are actually senior aged, 4 are junior aged, and 3 are youth aged (it would be their last year on youth if they were on youth teams). Next year, I will have 3 that are senior aged and 7 that are junior aged. If the age is changed to 13, then I would have 3 kids that are ineligible for my senior team but that need to be on that team because of their skill level and our lack of a junior team at that level. Not to mention that my very successful team of 10 would become a team of 7. I understand what you are saying about looking at the big picture, but quite frankly, I am fed up with looking at the "big picture" as it generally means "little gym, let me give your kids one more reason to look at the big gym down the road". It's our second year of having a senior team, and we are trying to build our program. It's bad enough that I have had event producers screw my team for being small- saying that they're not impressive because there's only 10 of them, their dance isn't visually appealing because there's not enough of them, or, my favorite- "it's your choice to have 10 kids". It's a team of 10 with full team tumbling and 3 stunt groups (2 are single-based). Please don't put the nail in the coffin and change the age to 13. Small teams have an uphill battle with some EPs to begin with.
Also, my impression was that the bottom age on senior 5 is because we are trying to keep the young kids who may not be mature enough for a senior 5 from getting hurt because of their inability to mentally handle it. Just my impression- we don't have a level 5. :) I don't see the same issue with having younger kids on a senior 3.