A few years ago Baseball divisions used funky non-descriptive names to indicate the age of its players:

• Atom
• Mosquito
• Peewee
• Bantham
• Midget

Because these names were non-descriptive and caused confusion among parents and players, they were abandoned in favour of much more clear age designations:

• Atom -> 9U
• Mosquito -> 11U
• Peewee -> 13U
• Bantham -> 15U
• Midget -> 18U

The "U" in these stands for "under," so "11U" means "children ages 11 and Under." This is different from "U11," which means "children Under 11." Hockey and Baseball use different age brackets, which is why a kid who is turning 11 in a calendar year would be playing on a 11U team in baseball, but on a U13 team in Hockey. Parents and coaches who participate in both hockey and baseball tend to put the "U" in front, but the officially correct designation is to put the "U" after the age in baseball.