Like you, I started studying karate _after_ our kids started. They eventually got bored at the blue belt/green belt level. I kept up, through second degree black belt level, and then let it slide rather than take the third degree test and all. I'd suggest that you go with what _you_ want to do, no matter what the girls decide. Telling your body to do something and having it damn well _do_ it is a real rush....

And it really helps when you have to write a fight scene.