If I didn't breastfeed my first children, can I breastfeed later children?

Yes. Your breasts produce milk with each pregnancy and the more you nurse/pump after giving birth the more you will make.
Yes. As intimidating as it is to be a new parent, it is not surprising that some opt out of breast feeding the first time around.Many of my mom's went this route & were very successful doing it for second and third kids. The hormonal changes will be there and you need only to harness your desire and get good coaching and things should go well.