I feel like you're opening the world up to a whole host of inter-dimensional boob physics related questions and I for one would like some God-damned answers.

Assuming they use "realistic" portal physics, the boobs would remain mostly stationary in the void, since the body wouldn't be moving much relative to the portal. Also, depending on what else is in the void, you wouldn't have to worry about gravity pulling them down either.

RAW, your boobs would never age.

You'd be 90 with the tits of an 18 year old.

I think there's no gravity and no air, do they would float around some. They would also be weightless I assume. I don't know how this works with things partially inside a bag of holding. I assume the forces would transfer to either side, but one side doesn't feel gravity. Maybe it'd be best to wear a bra under to keep things from floating around too much?

Well, you wouldn't have to worry about them getting in your way.

A bag of holding has much less momentum than it's contents would suggest. I'm guessing this would be the same.