Just wondering when your baby sat alone without support?

DS2 is 7 and a half months and is still nowhere near being able to. He's reaching all the other milestones, that is, rolling both ways, grabbing for toys, getting on his hands and knees and rocking, up on hands and feet like the 'downward dog' yoga pose, and is 'crawling' in his own funny style of dragging himself around the place to get to things (he gets up some speed too!) but he just won't sit.

DS1 was sitting just after 6 months. I know I shouldn't compare them but it's hard not to and I'm a little bit concerned.

Anyone have a baby who sat alone later with no health reason behind it?