The terminology is interesting, I would say it is sending him early or in the standard year, rather then deferring him, or I have heard people talk about 'holding them back a year'. I understand that you can (in Vic) choose to send early when born in that period of the year, but that the 'standard' year would be 5 turning 6. I was particularly young in the year and although I had no problems at all academically, it sucks turning 18 so long after everyone else (up to 2 years after for me sad). You also want him to be sufficiently mature to deal with everything in high school, which is difficult to predict before you are there. I think being so much younger and probably smaller as a result can be hard too, as wrong as it is, physical prowess can be pretty important among boys and 15 months can make a big difference. If a preschool teacher strongly suggested the particular child needed to go early, I would consider it, but not otherwise. Just about every teacher I have spoken to thinks it is best to wait as a rule of thumb.
Are you planning on him attending preschool? Have you thought about trying him in 3yo Preschool this year and that way you can choose to do 3yo or 4yo next year, depending on what the teachers and yourselves think is best for him? I was chatting to a preschool teacher at ballet the other day and apparently you can't do 4yo twice unless there is a diagnosed problem because of the government funding, 3yo doesn't get funding, so you have more options.
Good luck with the decision smile