Only time we got Elis age, from what I remember, is when Queen told him. She said something along the lines of the them not being able to have a discussion like this because he was only 8 years old at the time. Before he left. This is said at the pool at night, I believe. After Elis gets back home.
If I had to guess, their ages are:
Lia: 18 (She looked smol when Elis left. Though, that could be because of her being sickly. I would say she's 3 years younger than Elis, which would put her at 5 years old when he left.)
Elis/Elin: 21 to 22 maybe.
Cass: 23 to maybe 24 max. I think she's only 2 to 3 years older than Elis.
Conrad: early 30s. He might've just reached 18 when Cass and Elis pulled that prank. We also know he's not much older than Cass. So he would be 10 years old than Elis, assuming Elis left at 8. And be like 7 to 8 years older than Cass.
Aurora and Isis are both probably in their early 20s. I don't have a reason for Isis. But I do have one for Aurora. Since her father uses his daughters for political gain and marriages. Aurora being in the middle means that it would take time for her to be married off. Since he was likely waiting for a bigger pay off. So instead of being married off at 18 or 19. She was probably being held off until an Elis level person came around.