Your never too old to watch anime or read manga.
The I dont see the difference between movies like the fantasic 4 or spiderman, and animes like one piece. Just because one is animated and one has real actors with CG animation, doesnt mean one is better than the other. what matters is the story, and unfortunatly most people cant see past the Animation aspect of it. They think that just because its a cartoon, that its for kids. To them I say, you should check out death note. That is one of the best shows about a game of "cat and mouse" I have ever seen. I even got my girl to watch Full metal alchemist even though she always says she doesnt like anime. We were 22 at the time and I'm 26 now and still watching anime.
The trick to getting your significant other to approve is to show them that its actually a good story