I think Zelda and Pokemon both have demographic problems, just in a different way. Zelda on consoles as it is, appeals much more to older audiences now than younger audiences. For Pokemon it is the reverse. The demographics are definitely not balanced, so there are way less older Pokemon fans buying the games compared to younger Pokemon fans. That doesn't mean the audience isn't there, just that it is smaller.
For Zelda, we can see that when the portable entries on the DS appealed much more to a younger audience, being simpler, cuter, and easier, they sold much more in Japan, and still sold respectably worldwide. But with the console entries, the sales peak worldwide are the games which appealed most to a much more hardcore older audience - OoT and TP. When we look at the series sales in average, it's not really all that impressive compared with other multi-million selling franchises, especially those from Nintendo which actually attract a very wide demographic.
In the end, Pokemon's demographic problems are irrelevant because they have huge sales anywhere in the main demographic for the series, and there doesn't seem to be a shortage of children who are introduced into the franchise with each new entry. For Zelda, it seems like a bigger problem because younger audience are not connecting as well with the series, and when they grow up they won't have the interest in the series that the older gamers who did grow up with the series have.
For Zelda, we can see that when the portable entries on the DS appealed much more to a younger audience, being simpler, cuter, and easier, they sold much more in Japan, and still sold respectably worldwide. But with the console entries, the sales peak worldwide are the games which appealed most to a much more hardcore older audience - OoT and TP. When we look at the series sales in average, it's not really all that impressive compared with other multi-million selling franchises, especially those from Nintendo which actually attract a very wide demographic.
In the end, Pokemon's demographic problems are irrelevant because they have huge sales anywhere in the main demographic for the series, and there doesn't seem to be a shortage of children who are introduced into the franchise with each new entry. For Zelda, it seems like a bigger problem because younger audience are not connecting as well with the series, and when they grow up they won't have the interest in the series that the older gamers who did grow up with the series have.