“I am at present more impressed with the possible Evil that may arise to You from engaging yourself to him–in word or mind–than with anything else. When I consider how few young Men you have yet seen much of–how capable you are (yes, I do still think you very capable) of being really in love–and how full of temptation the next 6 or 7 years of your Life will probably be–(it is the very period of Life for the strongest attachments to be formed)–I cannot wish you with your present very cool feelings to devote yourself in honour to him.”
letter to her niece Fanny Knight
November 30, 1814 
I think they quoted this nearly directly in Miss Austen Regrets. Olivia Williams as Jane Austen.