I think I need to make a thread dedicated to Lux's age, just to point people to. >.>
Lux is, according to the official timeline right now, probably about 27 years old.
She is the only champion in the game that has a specific age mentioned in her bio, being that she was thirteen when she 'joined' the military. It is also mentioned in her bio that she ran espionage missions in Noxus during the Ionian War.
The Ionian War ended roughly 10 years ago (in 13 CLE, whereas this year is 23 CLE), which means Lux was in the military before then. I'm going to guess she had spent some time in it before that point, since she would have required some training before going into espionage (I'm also going to guess Demacia wouldn't attempt to send a 13-year-old into Noxus, alone, just before the war ended).
Therefore, we can rather safely assume she was thirteen before 10 CLE, meaning she's now more than 26 years old.