I've killed a thousand year old dragon and her children, who were, oh 930 or so.
They hadn't left the nest yet?
Well, they went to a local dragon college, and it was easier to stay in the home lair with mum[1] than board out, while supporting themselves (and giving some to the upkeep of the family home) with part time jobs such as fire-lighters, and then when they finally got full-time work they were lucky enough that there were quite a few maidens in the local area, and it didn't seem worth the trouble to move into a cave of their own and have to fully furnish it with treasures, when they were quite comfy in their own alcoves of the family cave. And, besides, mother's home-roasted knight was to die for. Not least for the knight.
[1] Mum was fairly happy about that. But not quite so happy about how they were decorating their sleeping area almost exclusively in jet items, deliberately drooping their wings and just grunting when asked to do any chores. But given the little girl dragons had previously gone through a big phase of Rose Quartz, Tourmaline and Morganite, surrounding loads of mirrors with all the pink jewellery they could find, and the little boy dragons hanging (mostly unoccupied) suits of armour from the ceiling and leaving their toy wagons all over the place, by now she'd gotten used to such fads. And at least they weren't freebasing shale-oil like some of the other tearaway adolescent dragons she'd heard about.