Some do and some don't. This is one of the big problems. The trapped players 2007/2006 are just starting their Junior year, they are in the same grade year as 2008, who can't play with the 2008 teams. A lot of Seniors are dialing back, or hanging it up, effecting lots of teams. It created a bad deal for the Juniors starting their year, who will be playing with 2008 teams next year.
The 2007/2006 teams are not a two year group. All the players ages are within a 365 day spread of being born. It is just referred to as two year groups because some of the Seniors where born in Nov-Dec of 2006.
It is good that the ECNL recognized the problem with Middle School, and started somewhat of a solution for them. There are still a lot of players not in the ECNL; think about all the players, not just a small few. That is how this problem started; focusing on the top 5%, at the expense of 95%. A lot of younger tournament happen during Dec-Jan, the trapped players would not be able to play in any tournaments, because they are "too old."
It's a US Soccer failure.