Possible Clues:

• Quick for one's age
• Informer admits resistance is full of vim and vigour
• Yare
• Like kittens
• Double agent hides recipe ¿ it's full of energy
• Active for your age
• Agile (for one's age?)
• Showing no sign of slowing down
• 'Agile, nimble (4)'
• (Of an old person) lively