The rush to close Rikers Island seems to be reversing at least one promising reform: the policy of detaining the under-22 crowd separately from older adults.

Back in January, the city started moving to close the George Motchan Detention Center to cut costs. Now Politico reports that this means the 600 male detainees who’d been held at GMDC are now in the general population elsewhere at Rikers.

That undoes the Rikers segregation of the young and old. Belatedly, the city Department of Correction has asked the jail-oversight board to amend the three-year-old requirement for age-based segregation. Mayor Bill de Blasio admitted Friday on WNYC that the policy proved to have “unintended consequences . . . we saw particularly — because of gang affiliations — some situations where it appeared to be making the situation less secure and making people less safe.”

But he also insisted the city will move back to the separation . . . eventually: “The goal is, as we move into new spaces soon, is to return that notion of having age groups separated again.” In other words, when and if those four new borough jails get built, the city will think about keeping younger offenders away from more hardened ones.

This isn’t the first time Team de Blasio’s been caught sidestepping a ballyhooed criminal justice reform. Three years ago, the Kalief Browder tragedy and a federal investigation prompted the city to adopt no-solitary-for-21-and-under rules. But last year, The New York Times found that eight young inmates had been transferred to an upstate jail . . . where they were placed in solitary.

Whatever a sensible, humane approach to the city’s jails might look like, it’s not what de Blasio is delivering. Even as the NYPD’s success in preventing crime has the detainee population below 9,000 (down by a fifth since 2013), the city spent a record $1.36 billion to run its jails in 2017 — with rising violence against both inmates and guards.

However pretty the handouts for the “vision” of the jails meant to replace Rikers, it looks like the current reality is getting worse. Better to focus on delivering improvements now — rather than retreating on the jail reforms you’ve already made.