Early 40's, no offense taken. .My guess is you are (please don't take it personal) in your 60's, or greater?
I really do think you're putting far too much stock in your personal observations about the continued popularity of Zep. There's only one objective measure, and that's music sales, which these days include streaming, which is probably the most accurate of all since it's counting each listen. Those numbers just don't support the idea that they are just as popular now as ever.
They still have many rabid and vocal fans. But by that measure, Slayer is the biggest band planet earth has ever seen.