How AP do you have to be to qualify as AP? How purist do you have to be to get the full (supposed) effect?
FWIW: With all DC we did things like co-sleep full time until past 2.5yo with most DC, 100% breastfed, babywear when practical, never believed in merits of crying. But also tried CC with DS1 for about a year (age 11-23 months).
Results:
DS1: started nursery age 9 months, cried solidly for an hour each time for first month (the nursery unhelpfully had a staff change the day he started so he was with strangers). Then he settled fine for 9 months, then he cried loads for 2 years (he was bored and wanted mummy). Then he settled easily at other preschools; ran into first day of school without looking back even though he knew none of his classmates and had barely met his teacher.
DD-started trying preschool when she was 2.5yo, she went to childminder fine before that: Fussed and unhappy and usually crying all the way until October of Reception (she wanted to stay with mummy). The preschool made special rules for her (what she was allowed to play with) because she wouldn't settle otherwise. (She's now my confident and outgoing child, btw!)
DS-started preschool at almost 2.5yo, went to childminder fine before that: Cried 10 minutes the first 3 times and settled easily every time thereafter. But hated school and took weeks to settle into reception.
DS-now 33 months: Not yet started nursery, I expect him to be a nightmare. He hates new situations. I tried him with a childminder for 7 hours (over 4 sessions) at age 15 months. Most of that time he was asleep; she asked to discontinue the arrangement as he spent all his waking time screaming inconsolably.
I humbly submit that personality counts for a lot in how well the separate from you, as much as anything else. My least AP-raised child is most confident in new situations and with new people.