Damian's excuse was a forced growth tank, which I have a great deal of difficulty seeing Cheshire having the means or interest in using on her daughter.

Lian's excuse, currently, is Death Metal: the fractured continuity that everyone is aware of, and which is full of discontinuities. When and if they get around to straightening that out, their solution can be that Lian has always been older than Damian: she was born in, what, the late 70s, early 80s, publication-wise; Damian was conceived in a 198…7? 1986? story; I forget exactly which year it was, but I'm pretty sure it was early post-Crisis.

Frankly, the single biggest headache to constructing a consistent timeline for the DCU lies in making the kids' ages make sense. My own inclination would be to start with the Robins, then figure out where everyone else fits around that.