I didn't defer my Feb born DD. But she was DD4 - due to 3 older sisters and the associated osmosis she was academically and socially advanced for her age.
Was only in state provision one morning per week, private the rest as I needed the hours. State nursery would have been against me if I'd pursued deferral as they had observed her tackling tasks much more in line with primary curriculum.
Was fine through primary, took varying composite classes in her stride and had friends over 2 year groups.
Now S3 and is the youngest in her friendship group (oldest of whom is a deferred girl a full year older) but you'd never tell by looking at the group their birth order. Subject prize winner in S1 and S2 so much more than "holding her own".
DD3 is a March birthday and tall for age (always taken for older than DD2) and much less resilient and confident in her social and academic skills. If she'd been 9 days earlier she'd have been a 4.5 school starter. I would have deferred her for sure. She'd have looked crazy in nursery but I wouldn't have cared.
I suppose the point I'm making is that I'd base my decision on the individual and how emotionally, socially and academically ready for school they seemed.
I realise DD4 still has the ordeal of prelims at 14 and actual exams at 15 to face but she relishes a challenge.