Yeah, I try to keep it loose so everyone can kind of see the characters as they'd like to. But I consider early-mid 30s for MC, and early-mid 20s for Nicki.
Some also asked me about this on Patreon, specifically regarding Gabby and Cassie. And since there's a couple other characters coming down the pipe that are in that same age group, I'll go ahead and say that, canonically, they're all 18. And I try to write them that way. Gabby mentions she has impending college plans, and MC mentions there's a significant age gap between him and Cassie, with her being born when he was already a teenager. In my head, he left home around 20/21, when Cassie was 5/6. So she's 18 now, and he's 33/34.
Earlier in the thread someone expressed discomfort with Gabby's age, thinking that she was written to be underage. I'll just say, I don't have any interest in doing loli stuff. For me, 18 year-olds already fulfill that young/innocent/naive archetype, while also presenting some significant moral ambiguity for the MC.