I read the first four DT books all at once in 1999, then read the fifth one just this year. I don't know if age has anything to do with it, but I find no problems today sitting through the slower parts of these books (and books in general) compared to my twenties or thirties. And I like longer books these days, as well.
You know, if they ever get around to releasing the whole DT series in one volume, I'd be tickled pink.
For those who might have been disappointed in the DT series, keep this in mind. Although known for his horror classics, Stephen King, IMO is not a "horror writer". IMO, he is simply a great author. Can he make you laugh as easily as he can scare the hell out of you? Yes. Can he spin a love story that will make you shed a tear? Well, if you've ever read 11/22/63, then you know he can. He can develop a character. He can make socio-political commentary. He can straight up gross you out and make you gag when he wants to.
I would say this... don't pigeonhold the man as a horror writer. Keep an open mind, and just enjoy his tales... because they're all intertwined.....
I think the first four books are some of the greatest pieces of literature that I've ever read. But, I do feel that the last three don't live up to their predecessors.
I personally don't think its an age thing but rather that King has such diversity in his stories. I enjoy hearing/reading about other people's favourite stories by King and, although IT and the Stand usually stand (pardon the pun) out, it intrigues me that people mention stories that would not have crossed my mind when thinking of my favourite.
I think everyone, for whatever reason, reaches out to different stories in different ways and I think your personal circumstances while reading also play a part of how you recall a story further down the line.
Personally, I read all of the DT stories while at university (between 2001-2004) apart from Wind Through the Keyhole for obvious reasons which is why I do not think the issue with the DT is an age thing. I too struggled a tad through Gunslinger but then the stories took over....could not put them down!