I was born middle of june, and Mum told me that my primary school didnt want to take me because I was so close to the cut off date. She had to make some special application to the school to get them to take me that year. So, I started when I was 4, and turned 5 in the June of that year. I was ready to start, and mum wasnt keen on having me stay back a whole year as the preschool I was at told her I was ready and it would be best to send me that year. I was able to commnicate well, I could read pretty well for a 4 year old and all that sort of stuff.
In primary school it was not really a bit deal that I was younger. The only annoying thing I remember was that in Kindy, everyone got a Dollarmites account with the Commonwealth bank, but you had to be 5 years old to have one, so I wasnt able to get one till later in the year.
There were plenty of people who were a year (or more) older than me all through school. In high school, some people were 18 months older than me. I have to admit, in year 11 and year 12, I probably was a bit immature... but I think it was also to do with the fact that school was just not a good learning environment for me. I didnt do well at school, but I excelled at TAFE the following year.

Even if the age cut off went by calender year, you'd still have up to a year in age differnce between the kids anyway? Theres really nothing much you can do to remove the age gaps. Also, if you move states, you end up with big age differences too sometimes, depending on where you are going to/from.

If preschool reckons your kids are ready for school, then they probably are. Theyre specialists in that sort of thing so they are able to assess it.