to get my AC to clear their old rooms?

I agree with Smileless they have been given enough time to clear your house, bin it.
I have always done that, once my children became teenagers anything left on the floor in their rooms or dirty washing not put in the basket went into black bags and was thrown out. They soon learned to tidy up.

I agree that you're not being unreasonable Dontaskme. When the deadline is reached, if it were me, I'd bag it all up, tell them I'd done so and if it wasn't collected by the end of the week (depending on how close they live to you), I'll be hiring a skip and throwing the whole lot out.

If they want it it's up to them to sort it out.

Good luck.

I've never cleaned their rooms since they were very young teenagers and I feel a bit shifty going in and sorting though their stuff to pack it
What you have to remind yourself, Dontaskme, is that these are not their rooms any longer; they are your rooms in your house, and you therefore have every right to go in them and do whatever you wish with the contents of the rooms!
No, YANBU

I moved anything that wasn’t obvious rubbish belonging to the DDs into the loft, it stayed there for many years.
When the GC arrived I eventually persuaded the DDs to collect the stuff they wanted and they went through it all, took some and threw the rest away.
Now the bedrooms ‘belong’ to the GC and now their stuff is in the loft!
I sense a pattern here......

I think part of the problem is conceding that they have the right to ban you from "their" rooms.
You aren't being unreasonable to declare a deadline and repossess your property.
As for heaving their stuff around and leaving a mess! All of that clutter should be binned immediately. They may take you more seriously after an assertive 'shot across the bows'!

Saying I haven't cleaned their rooms since they were young teenagers makes it sound like I live in a right s**t hole! I didn't clean their rooms but supervised and made sure they did. I didn't do their washing or ironing either. I was trying to rear independent capable people. I think I reared hoarders!!

We moved! Anything left was taken to the dump!

I've never cleaned their rooms since they were very young teenagers and I feel a bit shifty going in and sorting though their stuff to pack it, but that's what I'll be doing as they were given a deadline of the end of this month and its obvious they think I'm not being serious - BUT I AM!!!!!
Thanks for the advice. I wont throw anything away unless its obviously junk, and I wont take it to their homes as that'll cause more arguing. I will store it in the loft which is easier than some as actually the eaves. If I ever move then it will either have to be collected or it def will go in the skip. If I die and its still here as you say Witzend, more to clear . Wish me luck. I'll be texting deadline and consequences reminders to them later

They're taking unreasonable advantage of you. If they ignore your deadline, hire a skip and chuck the lot in it.
(I find this an interesting thread after reading one yesterday about grown up children asking parents to clear out hoarded stuff so, when the time comes, they won't have to!)

Get some boxes, pack the stuff up in some sort of system if possible (just in case they need to find something quickly) and write on the outside what is in it and who it belongs too. Then tell them to come and collect or you will dispose of them. Hopefully if they don't have to do the actual packing up they will make the choice to collect.
Don't store it for them because you will never be rid of it - my friend has recently moved house and even had to move her kids stuff that she stores for them even though they both have their own houses!

I would probably give a deadline by which time anything not neatly tidied/stored away will be gone. If there's stuff to go in the loft they will have to pack it and lug it up there. Those plastic boxes with lids are useful! But not too big, or they're so heavy.

Having said all that, we still have quite a bit of dds' stuff, and they are 38 and 41 and moved out long ago. But our house is more spacious than either of theirs. Apart from a load of books on shelves it's mostly tidily stored, and we often use their rooms for guests.
However, to me our house is still dds' (other) home and always will be until we're gone - when failing to get rid of their stuff will mean they'll just have a lot more to clear!

Every time you visit them take a box of stuff with you and leave it in their home whether rented or bought.

Mine have been gone a long time and I claimed the rooms back but I do have some of my sons stuff in the loft ,he lives abroad and keeps saying he needs to sort it but there never seems to be time if he visits.
Not unreasonable to want to use the rooms in your own house. Bag it up and do what you want with it they have had enough time and warning.

I would not even offer the space in the loft. If you give a long overdue ultimatum it is their choice. You ANBU. They are!