Cissnei's about Cloud's age since she's the youngest to have joined the Turks, and both Zack and Cloud joined ShinRa when they were just both teenagers. Who's to say that Reno didn't end up joining in his mid-late teens as well?
Nothing, Reno probably (though not certainly) did join the Turks when he was in his mid-late teens. When that event was in relation to everything else is what's uncertain. Reno is the third in command of the Turks before Before Crisis even begins. He's not a
senior Turk, like Zack can be a
SOLDIER First Class at a young age. There's nothing above him other Tseng and Veldt and not, unlike Zack, for a shortage of options. As such, he's probably been there for a while to earn the reputation he has.
I can see 26 but 33 is entirely reasonable as well. Cid's the one that's unreasonably young. All the same, the danger that they will make Reno look old instead of all bishi like Advent Children is pretty slim.