This is something that I’ve always wondered about with svtfoe. Particularly with season 4. Were the crew just lazy or something? I just genuinely don't understand why we didn't see an adult version of Star.
Because Star went to the Neverzone countless times, given yes, no where near as long as Marco, but it’s just very odd that they didn’t age her up, even the slightest. Especially when you consider how fast time flies in the Neverzone, it doesn’t make that much sense why Star stayed exactly the same every single time she went there.
And I mean if we're going by the so called "Marco's a 30 yr old trapped in a 15 yr old's body" belief, then it would be wise to age Star up in the Neverzone as well (even though Marco's confirmed to have the mentality of a 15 yr old when he leaves the Neverzone).
But either way, I reckon it just would have been an interesting plot point either way. Not just to see what Star would look like, but also reveal many new and advanced spells that Star would no doubt had developed over the years she spent in the Neverzone, potentially being useful for future events in like for "Cleaved". As well as help Star's character development by making her to become a lot more mature from her experiences that she remembers from the Neverzone.