Though I had an interest in birds when I was a young child (due to the influence of older family members), this disappeared completely when I was a teenager and was only rekindled in my late 20s for two reasons, that I can see. Firstly, technology (apps, ebird, etc.), and moving somewhere (away from the UK, in this case) with an affordable and extensive public transport system.
I think the second point is especially important. I don't sense a declining interest among young people, quite the opposite, and if there were greater affordable access then I think we would see many young people out birding.
On occasion I bump into younger birders, people in their teens and 20s, practically always on twitches but pretty much only locally in Madrid -for example, on the buses I already mentioned. I regularly return to England and attempt to bird there using public transport too and it's a pain in the arse. I spent five weeks in my hometown this summer and didn't even bother trying to get to some good birding spots in the same county because it's so impractical without a car. It's not like these places are in the middle of nowhere either, they're by large urban areas. Every couple of years I take a long trip to the US and it's a similar story there.
I work as a teacher and the teenagers I encounter who have an interest in birds typically tell me they don't feel as if they have the time to pursue it much, but I think that problem may be relatively unique to Spain where we have a particularly long school day with frequent exams. Although having said that, I could be underappreciating the time constraints on young people elsewhere.
Long story short: from my perspective there's a growing interest but also growing challenges for young people in many places.