Apparently, age 9 to 10:
Glenwright and Pexman presented five- to six-year-olds and nine- to ten-year-olds with puppet show scenarios that ended with one of the characters making a critical remark. This remark could be literal, aimed at a person or situation, or it could non-literal, again aimed either at a person (i.e. sarcastic) or situation (i.e. ironic). To illustrate: two puppets are playing on a trampoline, one falls on his face. ‘Great trampoline tricks,’ the other character says, sarcastically. Contrast this with two puppets playing on a saggy trampoline with little bounce. One of them says ‘great trampoline’, an ironic remark.
Research results here.
(Unofficial word has it that a 30-year old Midwesterner was left scratching his head: “But that’s not a good trampoline trick.”)