If you're trying to pin down New 52 Superman's age, think of him as a younger, still-finding-his-feet hero — basically late twenties to early thirties. The reboot trimmed decades of backstory and presented Clark Kent as a relatively new presence on the superhero scene: he isn't an ancient veteran with decades of exploits behind him, but a powerful, earnest guy who still feels like he's growing into the role. In-universe clues and writer interviews generally point to him having been Superman for only a few years, which pushes his civilian age into that late-20s range.
Reading the early New 52 issues — particularly 'Action Comics' and 'Superman' from 2011 onwards — you notice small timeline edits. Clark's college years and early career are compressed, his time as a headline-saving strongman is shorter, and the stories emphasize learning and mistakes rather than decades of seasoned wisdom. Some creators treated him as roughly five years into his career as a costumed hero; if you imagine he graduated high school at 18, went through college and early journalism, and then began publicly operating as Superman, that math makes sense for a late-20s figure. Different writers have shaded him older or younger at times, but the prevailing New 52 vibe is a Superman who's younger than the Silver Age or some of the pre-reboot incarnations.
There are always debates about exact numbers because comics like to be a little vague with timelines, and the New 52 itself sometimes contradicts earlier panels for story reasons. But for practical purposes — when comparing his attitude, relationships (especially with Lois and the Daily Planet), and the general tone of the books — I picture him as someone in his late 20s, maybe creeping into 30 depending on interpretation. That fits the storytelling goal: someone powerful but still figuring everything out, which I actually love; it makes his victories and setbacks feel earned and human.