Welp, he says he still needs to graduate yet talks about doing it with his gf. He didn't rebuttal the "wait till your 20" so he has to be younger than that (unless he honestly doesn't care how old people think he is) and his birthday was in February (because that when he got his birthday money, unless his dad is those parents that give birthday money months later :/) so he hasn't aged this school year yet. Due to all that speculation I honestly would have to guess that Duncan is 16 or 17 (unless he means graduating than I would put him at 18 or 19)