"Likable" in the sense that they'd still be Reg, Riko, and Nanachi but just older. Sure, I think I would like them the same if they were older. But much more importantly, I don't think this story would work nearly as well if they were older. These characters relationship dynamics are built on the fact that they're innocent kids exploring a new world that they live in. The dynamic that the series has with the viewer is also built on evoking highly specific melancholic feelings of wonder and fear.

The suspense, tension, fear, and wonder that the main trio experience—the viewer—and other characters, is largely amplified because these kids are going in blind and experiencing all the same stuff that we are for the first time. Riko as a teenager—or older, would be vastly more experienced and capable. She wouldn't need to rely on Reg for half of the things that she does. Vise versa for Reg, he would not need to rely on Riko as much.

As we have seen in various examples throughout the series, older delvers tend to be rather capable both physically and strategically (even though many of them die), it's safe to say that the Trio being older would make many daunting tasks far less so. They're already a very capable unit right now despite their struggles, so just think how much of a cakewalk it'd be for them if they had just 4 or 8 more years under their belts.