The wizards are getting hit with ageing spells. The old ones get young, and the solitary young one (Ponder Stibbons) is suddenly very elderly:
'Could be, sir. Er... some of them have gone, sir!'
Ridcully looked unflustered.
'Temporal gland acting up in the high field,' he said. 'Probably decided that since it's thousands of years ago they're not here. Don't worry, they'll come back when it works it out...'
Ponder suddenly felt breathless. 'And... hwee... think this one's the Lecturer in Recent Runes... hwee... of course... hwee... all babies look the... hwee... same.'
There was another wail from under the Senior Wrangler's hat.
'Bit of a... hwee... kindergarten here, sir,' Ponder wheezed. His back creaked when he tried to stand upright.
'Oh, they'll probably come back if they don't get fed,' said Ridcully. 'It's you that'll be the problem, lad. I mean, sir.'
Ponder held his hands up in front of him. He could see the veins through the pale skin. He could nearly see the bones. Around him the piles of clothing rose again as the wizards clambered back to their proper age.
'How... old... hwee... I... ha... look?' he panted. 'Like someone who shouldn't... hwee... start reading a long book?'
'A long sentence,' said Ridcully cheerfully, holding him up. 'How old do you feel? In yourself?'
'I... hwee... ought to feel... hwee... about twenty-four, sir,' Ponder groaned. 'I actually... hwee... feel like a twenty-four-year-old who has been hit by eighty years travelling at... hwee... high speed.'
'Hold on to that thought. Your temporal gland knows how old you are.' Ponder tried to concentrate, but it was hard. Pan of him wanted to go to sleep. Part of him wanted to say, 'Hah, you call this a temporal disturbance? You should've seen the temporal disturbances we will have been used to be going to get in my day.' A pressing part of him was threatening that if he didn't find a toilet it would make its own arrangements.
'You've kept your hair,' said the Senior Wrangler, encouragingly.
Ponder heard himself say, 'Remember old “Cruddy” Trusset? Now there was a wizard who had... good... hair...' He tried to get a grip. 'He's still alive, isn't he?' he wheezed. 'He's the same age as me. Oh, no... now I'm remembering only yesterday as if it was... hwee... seventy years ago!'
'You can get over it,' said Ridcully. 'You've got to make it clear you're not accepting it, you see. The important thing is not to panic.'
'I am panicking,' squeaked Ponder. 'I'm just doing it very slowly! Why've I got this horrible feeling that I'm... hwee... falling forward all the... hwee... time?'
'Oh, that's just apprehensions of mortality,' said Ridcully. 'Everyone gets that.'
'And... hwee... now I think my memory's going...'
'What makes you think that?'
'Think what? Speak up, you... hwee... man...' Something exploded somewhere behind Ponder's eyeballs and lifted him off the ground. For a moment he felt he had jumped into icy water. The blood flowed back to his hands.
'Well done, lad,' said Ridcully. 'Your hair's going brown again, too.'
'Ow...' Ponder slumped to his knees. 'It was like wearing a lead suit! I never want to go through that again!'
'Suicide's your best bet, then,' said Ridcully.
'Is this going to happen again?'
'Probably. At least once, anyway.'