Possible that they didn't know how or where it ran. What if it was headed back to a major SA base?
As I recall, they assumed the SA had formed an alliance with Earth when they initially arrived here.
As far as Millia's age goes... she's a front-line soldier that gets high-risk operations. I would assume 15 is OLD for her job, if she's been fighting since the day she came out of the clone chamber.
Compendium says Kamjin is 23, and that's the only other Zentradi I see with an age. It's safe to assume Britai, Exedol, and Bodol are a good bit older(both from art style, and Bodol's statements about culture in DYRL).
It's quite possible that commanders have durable bodies with extended lifespans(or immortal brains in a jar if they're in DYRL?) but the lower-level troops don't.
A fleet commander's experience is a lot more valuable than the guy shooting the gun, after all.
I've said this a few times already, but it's really a shame how little we know about the zentradi.