Our girl is 27th Feb birthday. Didn't defer.
High school was not a positive experience for her, turns out she's got ASD and leaving at 16 was a relief. School were very good, she was just not the right type of kid for high school because of her ASD, nothing to do with being young.
She got AAAAB at 16. Took a year out to work related to her uni course, partly because she is studying in England and so she'd have started at 16.5 with school leavers of 19, which is a huge gap.
I want to stand up for the "didn't defers" because if your kid's keen to learn and nursery says they'd be best moving on, then why hold them back? Someone's got to be oldest and someone's got to be youngest. Most of my peers raised eyebrows at me sending my girl at 4, but she was ready and keen. Primary school was a dream, high school was fine til S3 and then it was a disaster, year out was brilliant and uni is (so far) amazing for her.
Everyone makes the best choice they can for the child they have. If deferring works out for your kid then you will tend to recommend deferring. I didn't defer and it worked out so I'd always say "don't assume it'll be harder because they're younger"
Take your advice from nursery, they have a good handle on whether the kids will cope with the school system. A year to "play with" at the other end can be a real gift. Honestly, the only real issue we had was her being too young to drink in the student union (solution is to smoke weed in halls and drink coke in the bar, apparently)