Photo 12: LBBG (sub)adult female?, April 16 2001, Nachtegalenkeet, Maasvlakte, the Netherlands.

5cy LBBG are extremely hard to separate from full-adults and research has to make clear whether some features pointing to sub-adultness don't occur in adults as well (e.g. dark markings on the bill).  In adults as well, a reduced black bill-band may be seen and in spring the coverts may appear brown-hued in some full-adults. The features apply on this presumed LBBG.
The inner median coverts and upper tertials show a brown cast and almost all the lesser coverts are brownish hued as well.
The black markings on the bill may suggest last winter a clearer bill-band might have been visible.

Otherwise all characteristics indicate adultness.
In LBBG, males normally show a more robust jizz, stronger and normally have a much more aggressive facial expression than females. The delicate head and friendly impression this bird gives, suggests it's a female.

Whatever the exact age may be of this individual, it was holding a territory in the colony and probably mated this season.