While this dont gives an accurate age, since the story could have happened in the past, Garen was 19 in For Demacia, because of this:
How long had it been since Lux had come north to Fossbarrow?
She wasn’t sure, but guessed it was around seven years. Garen had just left to begin his training with the Dauntless Vanguard...
Since Garen's bio say this:
At twelve, he left home and joined the ranks of the Dauntless Vanguard.
So either Garen is 1-3 years old than Lux or the story is set in the past, but since it mentions the following:
Its fangs were hung in King Jarvan’s throne room, next to the newly-mounted dragon skull brought by his son and his enigmatic companion.
The story is set after all the Jarvan story, so Garen's entire bio, removing the last two paragraphs, happens in a span of 19 years or less.
I dont think that we have any estimative for Kat's age. If there is anything about Cassiopeia's age, we know that Katarina is the older sister, but I didnt find anything about Cassiopeia's age too. So idk.