Logan, I think you're right ... starting young probably helps. Thelma starting exposing her belly early on (I think to air out her spaying incision as it healed.) Once Weezie noticed all the attention Thelma got, she joined in.

Do Butter and Mimi ever lie on their backs at all? This does seem to be an invitation-only thing. If T&W are lying on their sides (not backs) and I try to sneak a belly rub ... I get smacked! The best time to try is when you come across a cat lying on their back in a sunbeam, all sleepy. Go down VERY slowly so as not to alarm them ... then try more of a tickle or rub than a pat. Be brief the first few times you attempt ... don't press your luck. Good luck!