A recent Guardian feature on similar all night metro services in
various cities showed an almost exclusively young clientel and not
many other people.

Hi from New York, where the unde, excuse me, subway has run 24/7 since
1904. Some services stop running after midnight, but every station
has service all night, most with 3tph or better.

In NYC there are certainly people coming home late from clubs, since
they can stay open until 0400, but there's also a lot of shift workers
and ordinary people doing errands they can't do during the day.

Everyone in NYC knows the subway always runs, so I expect the way
people use it is different from cities where late night service is a
recent or sometime thing.