Selina has no concrete age in N52 afaik. The only time I recall her dropping a number is when she goes undercover at a party during Winnick's run, and I'm sure she was lying.
All we know post-Rebirth is that she remembers the Wayne family from before Bruce's parents were shot, so it's safe to say she's probably very close to Bruce's age.
Her origin as Catwoman seems to be quite tensile. Rough assumptions based on context alone is all there is really