Hard to say since we don't really know the ages of both characters.
Last info we have on Barbara is pre Infinte Frontier, where it is said that she is in her late 20s.
Cass is based on Batman and Robin Eternals 15-17 (she was about 19 pre flashpoint), but that was before Rebirth ad Death Metal, so no idea how valid that still is.
Barbra (and Dick) are now both young adults, so Babs can't really have a teenage kid. The age difference between Cass and Babs seems to be about 10 years (Cass in her late teens, Babs in her late twenties).
Even Damian-Dick's relationship is now more of an older brother - younger brother, and Dick was basically Damian's father for a year (and Damian is much younger than Cass), so I guess we'll also get a similar mentor / older sister dynamic between Babs and Cass.
I just assume she and Steph are 16-18 while Babs is in her mid to late 20's now.
The age range was 28-35. And it also felt very weird because she wasn't portrayed as very mature in this run.
Also, didn't the writer say the entire final arc was out of continuity because she did certain things to prepare Babs to a new status quo that never happened ?(apparently to the 5G plans that were later discarded)
It's very hard to connect the last arc of her father hunting her as Batgirl with the Joker book in which he knows she's been Batgirl for years.
Also, the writer said that she didn't talk or consult with any of the other writers when she wrote the tie-in to the Joker War event - only with her and Nightwing's editor.
She also confirmed that there were a lot of changes that affected her writing.
I also think I read her saying that she needed to finish the arc in a specific way, and then the plans changed but her ending remained the same, but I can't find this interview now.
In the second issue of The Joker, Tynion references the events of Castelluci’s run and even has Babs ask her father about him taking aim at her, with an editor’s not similarly directing readers to an issue of the run. Also, James Jr.‘s death is a driver for Gordon in The Joker, which happened during that run.
So while it may be a little awkward in execution, it is meant to be in continuity.
In New 52 Cass was 10-12 when Dick and Babs were 16 which made her about 16 when they're 21
Moving into Rebirth, the back story were expanded from 5 years ago to 10 years ago, with Dick depicted as having been Robin since he was a preteen. Cass was still using her New 52 back story, which, since it's set during Dick's first year as Robin, should made her only 1 year younger or around the same age as Dick... but she's clearly not. She's treated like she's around Tim and Steph age.
So in Rebirth she's still 16-17 while Dick and Babs are 24
(Dick and Babs are aged 3 years just like Damian aged 3 years because their New 52 age, 21 and 10 respectively, were revealed early on in New 52 but remain stagnant during the duration of New 52... but Tim, Duke, Steph and Cass aged only 1 year because their ages were only revealed during the tail end of New 52, and the timeline details of Rebirth marked that New 52 end year as the same year as when Rebirth began... look, just go with this.)
Now, Infinite Frontier. Damian aged 1 year, meaning everyone aged 1 year. Confirmed by Dick also had his birthday, Jaime's graduating, and Jon Kent's entering college.
Now we don't know which continuity Infinite is using. I hold that they're using new continuity considering how many changes that didn't happen in either Post Crisis or New 52, but we got nothing on Cass so far, as far as I know
So for now, I just call that their current age difference is the same as before, making Dick and Babs now 25, and Cass 18.