Yes, it is confusing
In a nutshell, a child can start school as soon as they are 5 (actually on their birthday if they so desire!) but in any case, are legally required to have started school by their 6th birthday.
If your child turned 5 in March for example, they would be in Year 1 for the rest of that year, until the end of the school year in December. When they go back to school after the summer holidays, they would then be in Year 2, and so on.
If your child turned 5 in October on the other hand, they would be in Year 0 for the rest of that year, until the end of the school year in December. When they go back to school after the summer holidays, they would then be in Year 1, and so on.
This means that some classes will have mixed 'years' and mixed ages in some cases (our daughter was only 7 in March, but she has some children in her class who are nearly 9), but it all seems to work itself out in the end!
HTH