I can fully believe that an 8 year old has middle school friends, however - I think that there will be a natural gap as they age. Dd21 is PG and loves intellectual conversations. She adores talking to her brothers. But, honestly, would she have ever had close friendships with someone more than a few years younger in HS? No...
Sure there are gaps, but for some kids it doesn't matter. DD isn't looking for close friendships. She just looking to hang out. It works for her in that she has friends her age too and it never gets dull. When you're subject accelerated it works out because you can do both. Go hang out with aged peers playing hide and seek and go to a horror movie with your older friends because they are ready to do stuff like that. Actually she scares some of them because they are not ready for that!

she had a sleepover here with 3 middleschoolers and they were run ragged and one couldn't sleep for a week because she was scared of the movie. DD was surprised and now wants to find some high schoolers to hang with.
But close friendship..who knows. It doesn't matter really. They tell her who they like..what boy is cute..she keeps their secrets. It's funny really..an 8 year old texting 14 year olds about their crushes and other fluffy topics. DD just goes with the flow and doesn't take it all seriously.
But when it comes to math she just can't sit around. She doesn't want to go at the pace they present math at school. She just wants more and in that subject she can get that acceleration and the social issue really isn't an issue for her so there are no cons for her as far as being subject accelerated.
It does depend on the kid.
I get what you're saying too. It doesn't matter when you get calculus. If you're kept busy and are happy doing other things, then that's great.