There was a bit of confusion about this at our first meet of the season. The age you compete in for 2014 is the age you will be on the 31st December 2014. For example, if you have already had a birthday this year and turned 7, that is your age group for the rest of the year. If you turn 7 later in the year, even on the 31st December, you are also in the 7yrs age group.
For the older juniors, if you turn 13 or 14 this year, you are in the U14 age group, 14 & 15 are U16s and 16 & 17 are U18s.