I have not seen that in SD (with 1 exception that I am aware of). Not saying it isn't happening, I just have not seen it. Not sure why the parents of an '05 would allow it (unless we are talking ego again).
Unless we are talking about the rare unicorn, the '05 playing '04 academy will get limited playing time and would be playing against bigger, faster, stronger girls. Yes, she gets to practice against great competition, but practicing isn't that same as playing in games that matter. And I would be willing to bet that the 25% she does get, as mandated by US Soccer, will come in garbage time.
If the club is pushing it it makes me wonder what's happening with their '04s; they should have enough talent to complete the roster without dipping into '05. If not, what does that say about the club?
If the parent is pushing it, and the club is allowing it, it seems like a money grab and the club tying up an '05 player a year earlier at the expense of their development.
Barring the very rare '05 that can no only play with '04s but stand out against '04s, it doesn't make sense to me. My $0.02