Well, to update, last night after work/school I asked DS15 to put on a white wash. Without any further explanation or complaint, he went upstairs, sorted the whites in the laundry basket out, brought them down, put them in the machine, popped the washing gel stuff and fabric conditioner in the correct places and asked me whether to wash the load on a 50 delicates or 60 cotton.
I asked DS13 to empty the dishwasher and, later, to transfer the stuff from the washing machine to the tumble dryer. Again, with no complaints or explanation, he did both, except to ask which programme to use on the tumble dryer. When the tumble dryer beeped later on, he got up (from watching TV), emptied the clothes into a basket and then asked what he should do with them. I showed him how to fold them (doing the majority myself while he looked on doing nothing, but hopefully learning).
Last night at bedtime I did have to tell them to go up to bed, and I did remind them to set their alarm clocks. This morning I did not remind them to get up, or washed, or showered. They were both up, washed and dressed in good time and made their own breakfasts, as they usually do, then stacked their own stuff in the dishwasher and DS15 had the sense to put it on a rinse cycle. DS13 did leave sticky juice cartons on the table but, well, that didn't seem too bad.
So it turns out they are not totally useless after all, and that I have not totally ruined them by doing things for them up until now. Thank goodness I realised quite how much I was doing before it got too late.
I've thought about it and cooking, cleaning chores and bed changing I am going to leave until the Easter holidays, since they are both away at half term, and then embed as holiday and weekend rituals.
Thank you all so much for responding to my post and making me realise that I need to spend the teenage years making sure that they have some everyday education and abilities to go alongside their academic, sporting and other successes/interests/life preparations. Hopefully I will stick with it and ensure that I turn out two young men who are responsible and aware in most if not all departments.
:)