How can I get my future husbands teenage kids to help clean the house?

My future husband goes to work at a good, but strenuous job all day. His kids are teenagers (18, 15, and 14) and just make a mess everywhere!! Bathroom, Kitchen, Living Room! I am the one that cleans up everything everyday. My future hubby does do a share of the cleaning when he is able, and I give him major props for that. But with his kids being teenagers, I feel they should be able to do a load of dishes or vacuum here and there. I have tried to discuss this with him. He simply says "they will learn later in life"..but why cant they learn NOW?! I don't want to sound like a nag, because that's not my intention. I'd love to have his children help out a bit around our house. It is very bothersome to me and I feel like my cleaning is for nothing and under-appreciated. Any advice or tips you can give me to help me with this a little bit?