I'm pretty new to the cheer world - as you can tell by my handle :) - but I've been involved in youth soccer as a coach and official for many years. I've been reading a lot about this topic on this board and wanted to share my insight, since playing on teams above one's age level ("playing up") has always been a topic of much debate in programs I've been involved in.
Now I've seen kids thrive when asked to play on a team with older kids. Generally these are kids have a few key traits:
1. They're able to excel at the higher level, rather than just be a contributor. Kids tend to develop faster when they're leaders on a team rather than just role players. Our rule of thumb was that if the player was just "good enough to make the team", then they were better off staying at their age level.
2. Physically mature. Practices and games are longer, and tend to be more physical. Just because some has the skill set to compete with older kids, it doesn't mean their bodies are ready to take that kind of pounding.
3. Emotional and social maturity. There are many reasons why even a talented, athletic kid might not be ready to play up. As you move up the ladder, games and practices become more intense. Coaches expect more from their players, especially on competitive traveling teams. Younger kids might not be ready to handle that. As well, kids who play up tend to struggle to fit in socially with older teammates.
There are always going to be prodigies that need to be on elite teams at a young age because their skills demand it. You don't want to keep someone on a team far below their skill level to the point that they're bored. But of all the kids that I've seen play up over the years, I'd say that about 70-80 percent of them had no business doing so, and in many cases it hindered their development and in some cases ushered them out of the sport. That didn't mean that they weren't talented players with a lot of potential, it just meant that they weren't ready to be on the team they were on.
And keep in mind, we're often talking about having kids play up maybe one or two age levels - for example, a 9-year-old playing U12 instead of U10. I wonder how these 12-year-olds that end up on these senior level 5 teams fare, when they're a preteen facing the pressure of big competitions on teams with girls four, five or six years older than them. I'd be curious to hear stories from cheerleaders or coaches that went through that, how it worked out, and what lessons were learned.