I think crossovers should only be allowed to one level... in other words, you could do Jr 2/Sr 3 but not Jr 2/Sr 4. I feel like this issue has been:deadhorse: on these boards. We all know the arguments for and against it. I will re-iterate my personal feeling that a Level 4 or 5 athlete can not only compete Level 2 or 3 skills with more technique, but they are also going to be much more well-conditioned athletes. So, yes, it is an unfair advantage.
My prior gym, we had our Junior 4 lose 2 kids in the middle of the season. We had a Senior 3 and a Senior 5 with age-eligible kids on it who very well could have crossed over, but at that time we had a no-crossover policy. They competed with 18 for the rest of the season, and did very well. If your team is comprised right to begin with, and your coaching skills are up to par, it shouldn't be an issue to compete a team that may not be maxed out on numbers. If you are at 12, 14, 16, 18, and you can't do well, then maybe that team wasn't put in the right division in the first place. JMO :)