The full hood in Black-headed Gulls should be more correctly called 'breeding plumage' rather than summer plumage. In early March it's still just about winter, yet many birds will already be pairing up in readiness for breeding in the coming months, if they haven't already done so. This is why many species are looking their best at this time of year (from the ducks on your local lake, to the Blue Tits in your garden: it's not just gulls!).
I've been seeing the odd very early Black-headed Gull start to re-acquire black hoods well before Christmas (and the odd one even from early November!). At present perhaps a quarter or more have a full hood, with many others in various states of moult. Lots however, are still in full 'winter plumage' but are likely to acquire it relatively quickly as peak breeding season approaches. It's a process that I believe is triggered by sex hormones in the body, and there is a very broad 'window' when individual birds begin to moult. I agree though, that it seems odd why some individuals develop hoods so early when it is clearly far too early to even begin courtship.
Towards the end of the breeding season, you will notice that many adults will have lost their hoods quite quickly (by mid-late summer) even though it is technically still summer, and some might even be feeding dependent young. No need to maintain appearances once the hard work of attracting a mate and raising chicks is done!
Or perhaps even more, 'courtship plumage', as that's what its purpose is - they don't actually need it while incubating eggs, or raising their chicks, even though they do generally still have it then.
This is more obvious with Royal Tern, which generally only has full 'summer' plumage for a month or two in March-April; by May, many are already getting back into 'winter' plumage when they are still early in the breeding cycle.