Possible Clues:

• Nonlearner, in a proverb
• One who can't learn new tricks, they say
• Set-in-ways one, supposedly
• Unteachable one, in a proverb
• Poor learner, according to a proverb
• One can't learn new tricks, it's said
• Hard one to teach, in a saying
• Unteachable sort, in a saying
• 'You cannot teach an ... ... New tricks'