I wasn't there when the kids were really young, but I picked up on issues right away - first date type of right away. O clung to me through the haunted house like I was her BFF, and she'd met me half an hour before (age 8). J barely talked (age 5).
husband said he noticed O at 18 months, when the tantrums started, and they had to restrain her. Apparently he would tell her pre-K, K and 1st teachers that she was manipulative and they'd blow it off - "Oh, I understand kids like that"... Then come the first P-T conference they'd shake their heads - "You were right... She's GOOD at manipulation."
J - he didn't seem to hear anyone well, talked late (2 years from what I hear) and even then not much. I still think, in addition to the Fetal Alcohol Effects (FAE), he's somewhere on the Autism spectrum, probably Asperger's, but hey, I'm not a doctor. When I first met him, he barely spoke. Still didn't, until I'd been with husband over a year and J was spending more time with us. I refused to do the baby talk everyone else used with him - and he would talk to me. Not O, or husband, or mother in law, or father in law - but he would talk to me - but strangely, only on the phone. Where I couldn't understand 90% of what he was saying because he talked SO FAST. But still, not MUCH talking.
After a meltdown when he informed me that husband and I COULD NOT GET MARRIED because we would start fighting, he started talking more and more. And now he talks "to himself" - but only when someone else is in the room...