Amusing! No one seems happy with authoritative dictionary definitions, and prefer their own or someone else's speculations.
The male equivalent around here in polite conversation is "toy boy" since in many cases the female is older. The term gigolo is seldom heard in America anymore- my daughter wanted to know what was meant by the song title "Just a gigolo."
There is no evidence that "tart" came from "sweetheart," but it is not unlikely; it also could come from sweet tarts. "Applied (originally endearingly) to a girl or woman (often one of immoral character." (OED) This suggests that the word was not derogatory when first applied. Tart in the sense of pastry appeared as tarta in medieval Latin, and was in English before the 16th C applied to fruit as well as to meat tarts. Tart in the sense discussed here first appeared in print with regard to a court case in the Morning Post newspaper, 1887. "The paragraph... referred to the young ladies in the chorus at the Avenue and spoke of them as 'tarts.' It was suggested on the part of the prosecution that the word 'tart' really meant a person of immoral character." (OED) This passage suggests that the term is older, but has not yet been found in older writing.