If you have some time to think about it, then perhaps wait a while and see how you feel by the deadline.
But, in your situation, I think I'd be very tempted to defer her.
My DD is an August baby, and when she started reception 15 years ago there wasn't the option to defer. If there was, I'd have taken it.
She wasn't a particularly young child, but had just turned 4 when she started school. Friendships weren't particularly an issue - her best friend was nearly a year older than her - but academically there was a big divide. It was very clear that the 'top table' was made up of all September/October/November children. It was almost a self fulfilling situation, as they did better, they were given more challenging work and the gap became wider.
This does eventually narrow, and my DD went on to do very well, including attending a super selective grammar school, and is now at uni after a gap year. But it was definitely a disadvantage being that much younger than some others in her school year, and statistics show that this disadvantage remains through their whole school lives.
It's the same for sports - younger children are obviously on average smaller, less strong than their older counterparts, and are therefore at a disadvantage on the sports field. This can affect confidence/interest in sport that can last a life time.
Socially, particularly in sixth form, her age became a factor. As she was the last in her year to become 18, she was left out of many social occasions. Her closest friends made sure to arrange some social events that she could attend, but there were many trips to the pub/clubs that she just couldn't go on.