Of course the campaign has been popular online. It's all about demographics; the kind of people that find this online are those that will usually like it.
Now think of a not-so-educated 60 year old tourist going to see a nice park on his vacation.
It is indeed about the demographics - I don't think the age is a great factor, but it fits UK culture very well. In fact, its innate Britishness could well appeal to older age brackets more.
It's not a campaign to convince elderly tourists to return to the park.
It's a campaign to convince people like me, a National Trust member who's half the age of the person in your example, that the National Trust attractions aren't all stuffy and boring and the NT wants you to actually explore and enjoy them.
Now think of a not-so-educated 60 year old tourist going to see a nice park on his vacation.