In the New 52 comics, that's the way it happend. Her pod arrived years after Clarks did, but she was still the same age as when it left Krypton. I think they stated it circled our Sun afew times before it finally crashed onto Earth. That could be why she remained the same age as when she left Krypton, that or something in the pod itself kept her from ageing.
As for her age being 24 in the show, I think that's their first mistake already. Kinda missing the point of "Supergirl" when she's no longer a "girl" and more a woman. So many good story arcs going to be missed out because of this move. Maybe they just want to be allowed to make her more sexual as a 24 year old, instead of say a 16 year old. But then, this isn't supposed to be a Powergirl show.
Aside from that, I'm really looking forward to seeing this take on the character.