Why won't your youngest leave for at least 10 years? Were you in your 40s when you had him/her?
I have to say I don't envy you. My parents passed (about 12-13 years ago,) when DD was only about 11-12, and DHs passed several years before, and whilst it was tough at the time (losing both sets of parents within 7 years, and us both only being around 39-40,) it's something of a blessing now, because we have no elderly parents to care for or worry about.
We did the caring and helping them in the last 5 to 7 years of their life, and DD was little then too, and we worked, so it was tough. AND tough losing them when we weren't hugely old.
DD is mid 20s now and left home some years ago, and we don't have any caring duties now. Not for anyone. Both in our early to mid 50s.
No help to you whatsoever sorry @AnuSTart but I do empathise. It's tough caring for parents, especially when you have children, and you work too, and it's a relief to have no more caring duties now. It will no doubt come back around though if DH becomes infirm, or DD has a baby and we help look after it. (Although the latter is not mandatory.) I suppose caring for DH is not mandatory, but it would be a bit mean to not do it!!!
Also, am I correct in assuming your DH want YOU to look after HIS parents? LOL fuck that!