There should still be the flexibility to have U and grade based games. The U designation should be confined to a one year range; just stick to a universal 12 month DOB range (there will always be older and younger). Grade based should also have some age stipulation and allow for maybe an 18 month DOB range.
The original reason for a 2 year U span was to allow less developed areas to combine 2 grades and field teams. This policy has been manipulated and folks have worked the system to gain an advantage on the competitive side of the sport. The 2 year rule is a very good rule for a rec in-house league that has low enrollment and is developmental... but it losses its efficacy during competitive lacrosse.
If you are in 7th grade, but you're 18 months older than the youngest eligble player (you really should be in 8th grade)... you play up (simple). You can cause no undo harm to others, you need to prove yourself against your true peers.