My two were 4 and 6 when placed with me. I knew I wanted older children for various reasons so wasn’t worried about the “missing” years.
My DD6 had developmental delay so presented as younger - more like having twins than two siblings 2 years apart. They are 12 and 14 now.
They called me mum from the outset but it was a name to them, rather than a relationship if that makes sense. Mum is what you call the female carer so that’s what they did. That changed over time as they realised they were with me full time. I think they needed to be with me longer than they’d been anywhere else to feel properly secure, which took a while.
It depends on what you mean by forming attachments. Attachment (as in attachment theory) is a longer term process with all adopted kids. Very often when people talk about attachment they mean bonding or relationship building. In terms of bonding that took a while for me with my DD, for various reasons but that can be the case with adoption and with birth children. You do need to be purposeful in your intent and your caregiving though - really making time for connection in a way you might not for a younger child where care is really needed in a very practical sense.
Attachment is an ongoing process, I’d say her attachment style with me is pretty secure now but it’s taken a lot of work.
In terms of being kids, we still enjoy family days out, cuddling on the sofa etc - it changes as they get older and it’s very much on her terms but she still needs her mum. I still sometimes read with her - we usually have a book we’re both reading and sometimes she’ll ask me to read a chapter to her. We did bedtime stories until she was 10, sometimes me reading, sometimes her reading. Yes the time they are small is shorter but you’ll develop things that are meaningful to you both.
I do feel I’ve missed things with both kids, lots of firsts I didn’t get to see. But there are other firsts - her first school party, her first school show, the first time she chose clothes to buy, our first hair cut and lunch out, the first time she went out independently etc. They all have their meaning and if missing her first 6 years means I get the rest of her life, I can live with that.
Her birth mum might have seen her first steps, and her foster carer might have seen her first day of school but I get to see the amazing young woman she is growing into. She knows me as her mum - always there for her, on her side, caring for her and cheering her on. She knows she has other family out there, and that’s hard because she has memories of them many of which aren’t good. But she knows she’s mine in every way that counts. I can live without her first 6 years in exchange for that.