In Star Trek Voyager the character Seven of Nine is supposed to 24 years old when "liberated" from the Borg by Janeway et al.
She was assimilated by the Borg in 2356 at age six, along with her parents, but was liberated by the crew of the USS Voyager in 2374.
This, however, doesn't seem to jive with when the Federation became aware of the Borg. The Federation is first exposed to the Borg in Q Who?, which took place in 2365. This puts Seven and her parents being assimilated 9 years before the Borg and the Federation were introduced. But if this was the case, then the Borg would have "been aware" of the Federation prior to Q Who?, which would be at odds with Guinan's warning to Picard.
Reflecting upon events in Ten Forward with Picard, Guinan says that the encounter with the Borg happened before it should have, and for the moment, the Borg are only capable of seeing the Federation as "raw material to them". And because they are now, Guinan begins, "aware of our existence," "they will be coming," Picard continues. Guinan ominously says, "You can bet on it." [emphasis mine]
The only explanation that keeps these details from countering each other is that the Borg had been attacking the Federation prior to Q Who?, and for some reason the upper echelons of the Federation and Starfleet kept this threat hidden from the Captain of the current Starfleet flagship, which in my opinion seems unlikely. Therefore, the only remaining explanation is that Seven lied about her age for some reason. If this is the case, then how old is Seven of Nine really?