This is an interesting and challenging question that can't be answered authoritatively, but the context of the play provides us with fairly reliable clues. Cloe, the lustful shepherdess, would appear to be be the oldest. She worries in Act I, scene iii that she is "too old." I would put Clorin in the middle, as she has spent time mourning her dead lover and living in a sacred grove by his grave. She has developed skills as a healer, indicating some maturity. This would leave Amoret as the youngest. The fact she is so innocent bespeaks youth, just as Cloe's lustfulness indicates age, and though Amoret says in Act IV, scene iv that she has been "wooed by many," she still comes across as younger and less authoritative than Clorin, who has spent time mourning the dead lover and developing her healing arts.