I get nerdy about little details like this, so here's my take: the movie never actually hands you a number for Hobie Brown. In 'Into the Spider-Verse' he shows up as the punk, guitar-wielding Spider-person with that anarchic energy, but the script and onscreen captions don't list his age. That leaves us to infer from cues — his clothing, his attitude, and the way he moves through the montage of Spider-heroes.
Putting those clues together, I read him as being in his late teens to early twenties. He reads like someone who’s old enough to have a fully formed scene identity (the DIY punk vibe), but young enough to still be reckless and anarchic. Comic versions of Hobie Brown have sometimes been portrayed as a young adult too, which lines up with the film’s silhouette. Personally, I like imagining him around 18–21 in that universe — just edgy enough to smash a guitar and taunt the villain, but not so old that he loses that scrappy, rebellious spark.