I think this is certainly a Roseate Tern. In addition to the features discussed by others, I think there is single diagnostic feature which has not been pointed out by others and that is the pattern of the outer tail feather. In Common and Arctic terns the outer web is black and forms a very narrow but obvious black line along the outer edge of the tail. In Roseate Term This is white and contrasts with the rest of the tail which is slightly greyer. This is well shown in the photos.
Regarding age, I think this is probably a first summer rather than an adult. I think that given that wing moult has only just started (inner only primaries being replaced) it seems unlikely that body moult would be as advanced as this (i.e completed) if this was an adult. The bill colour and leg would also be very advanced into winter 'plumage' for an adult.
A very interesting bird. Thank you for posting the photos.
