I don't think so. My kids are one and three. I've been reading mythology to them since they were born. They seem to enjoy it as much as any toddlers enjoy books.
I figure that, even if they don't understand the meanings or morals of the myths, it's still nice to read them so that when they get older they will have an interest and already know several of the myths. If parents can start teaching their children about religion as babies I figure, why not mythology-as long as they're PG of course.
I prefer myths to fairytales anyway. Too many fairytales are centered around a poor helpless, but ridiculously beautiful woman, who needs a man to rescue her. All the man has to do is crook his finger and she agrees to be his wife. They go off and live happily ever after. Stories like that get my goat and I prefer to not even read them to my daughter. The last thing I want is my girl growing up thinking that all she has is a pretty face, though she's a beauty, and that she can't offer anything to the world; that she needs a man in her life to survive and be happy. I'll take fire-breathing dragons, babies with wings, or a king with asses ears any day.