Thanks, Colleger, LAlady, kilmuir,TalkinPeace2 & radun10, for your sharing.
Perhaps, it may help others to understand the situation better if I give more specific details.
DS was identified as Gifted when he's 5 while us in Australia & he's then recommended to jump 2 years. My wife & I eventually agreed to let him jump 1 year. So, since the age of 5, DS's been attending a class with classmates a year older than him : between 2008 & 2010 in Australia, and from 2010 onwards in UK. DS's always been popular amongst his older (and younger) peers as he's always 'Helpful, Humorous & Kind to everyone' according to his headteachers & class teachers in Australia and UK.
Also, we've an older son, already attending the grammar school DS's applying, who's 3 years older than DS. They always have a good relationship - another proof that DS's ' Socially & Emotionally Mature enough to get on well with older children.
Moreover, my wife & I are experienced Paediatric Nurse & Police Inspector respectively, both have studied Child Psychology during our ongoing training & come into contact with hundreds of children during our long professional life. In addition, I helped in classroom for about 3 years in Australian primary schools. We therefore do have a Very Good understanding of children development, knowing DS's Socially & Emotionally Mature enough in getting well with older children.
Furthermore, DS's already at Level 4A in ALL English areas & 5B in Maths & Science in Key Stage 2. (KS2 expects a Year 6 students to achieve Level 4 by the end of the Year 6, that is when a child is aged 11) And DB's consistently getting a high score in the 11+ Practise Paper (higher than his elder brother's, who was accepted by CGS 2 years ago) In fact, DB's primary school headteacher had hinted last May that our younger boy could have a go at 11+ LAST Nov but we decided to wait for this year.
The situation is now a bit tricky. The grammar school headteacher is more than happy to let DB to take the 11+ in Oct. It's the council considers he's too young, may not be Socially & Emotionally Mature enough to start secondary education. The primary school headteacher just E mailed me he DISAGREED with the council, and he's to write a Strong letter of support, arguing it's in the best interest of our DS to start secondary education in 2013 rather than being further held back.
So, it's NOT a question of I want, or what we want, but more about What our DS NEEDS.