According to her article in thepeerage, she was born ca. 1773. She would be 40 when this portrait was done. But this is a very youthful 40 so Mee portraits may not be especially trustworthy as far as accurately depicting people is concerned. Portrait of someone purported to be Louisa Tollemache, her mother, are here and here.
From liveinternet.ru/users/4945357/post298629422.
This portrait leaves something to the imagination, but not much.
Keywords: 1813, Mee, Catherine Sophia Manners, Lady Heathcote, Manners family, Heathcote family, British, long curly coiffure, square neckline, quarter length close sleeves, Empire waistline, close skirt