Sometimes you can age birds more precisely than just immature/adult. According to Sibley, Little Blue Herons are white from fledging through winter into the next spring. Then the bird moults to adult plumage, so this bird looks like it is in its second year and is a little less than one year old. All this underlines how important it is to post dates with photos. When was his one taken?
Thanks, John, that agrees with my investigation as well. At first I thought this was a life bird but, now that I think of it, I think I saw an adult LBH in Ding Darling in March on Artie's IPT. It's still neat - I have not seen one of these up here in NYC until last month.