The movie may have been released in 1986, but the Disney English version in which Dawson plays Pazu wasn't recorded until about 12 years later. So roughly he was around 21-ish or so when he did the part. Which is a rather curious choice considering the character is roughly around 13-14 years old. It's only distracting if you're accustomed to the Japanese version (who sounds younger but also sometimes too aggressively screechy), but that's just to answer the question.
(Oddly, Pazu's Japanese voice actress was 31 years old when she voiced the part, too.)
No problem. I am really fond of this movie, and while I may be in the minority here, I think the Disney dub is really good. Sure, Pazu and Sheeta sound a tad too mature, but they're competent IMO, and otherwise the performances are great.