Shrink Wrap - S4-E25
Corrected entry: When Carrie and the psychiatrist are at the birthday party in her flashback, she says that the cake he is eating is "like forty-two years old". If this flashback took place forty-two years ago and the young Carrie at the party appears to be around eight or nine, then adult Carrie would have to be at least fifty years old.
Correction: I saw this episode again recently, and I thought I heard her say "like twenty-two years old." Which would be accurate for her current age, early to mid-thirties.