its just that when u look at the very beginning of the season 1 episode 1 Celestia is talking about what happend with nightmaremoon blahblah and she has pink mane there, and she was "adult" at that time or? maybe she was not, as u suggest... but why Luna´s mane hasnt changed a bit then? (or it did? havent i noticed something?)
maybe the reasoning was the part when she said "Luna, u must lower the moon!" and Luna was like "Nope, im gonna hunt u down with mah laserz