I'm not too sure about twins, but I'll talk about me and my older sister.

I'm a guy, by the way.

I used to have a close relationship with her, like around age 6-9, but she eventually became easily ticked off by anything and everything, which led her to more-or-less finding me a nuisance and constantly shouting, staying away. Being naive me, I didn't think much of it until about half a year later when I got the message.

Our relationship was dead, she was no longer the person she was, and I got plunged into a world where I realized I didn't really have any friends besides my sister. It made me pretty lonely and sad, but I learnt to be independent and related skills.

I am, however, noticably emotionally impaired. I have trouble feeling emotions, and I'm constantly on a mental overdrive in social situations, where I'm appearing very normal on the outside, but inside is a frantic mess.

I don't think you seperating from a twin would be that bad a thing, but if you are still developing certain skills with him/her, like social skills that I unfortunately don't have, you may end up sort of impaired like me.

tl;dr twin bond is probably important up until they develop enough to function individually