In the episodes of Dragon Ball Super, how can the voice acting on Goku be the same after 18 years from DBGT?

She seemed to do just fine on this panel three years ago.

I have been a vocalist for more than 50 years. I have sung baritone/bass since I was twelve, and only recently (in my 60’s) has my range been affected by my age. The well cared-for human voice, whether speaking or singing, can withstand effects of aging that are more apparent on other parts of the system. A good example would be Tony Bennett. Apart from some stylistic changes, (and some improvements in technology) the voice in this relatively recent recording is almost indistinguishable from this recording from the 1960’s. Other artists that come to mind are Aretha Franklin, James Earl Jones, Charleston Heston, Patrick Stewart, and many others.

To add to what @Yetanotheruser said, voice actors are actors. Nicole Kidman doesn’t just walk onto set and act like herself. She transforms herself into a character. Similarly, voice actors don’t just walk into the studio and talk. They transform their voice into that of the character. So even if Masako Nozawa’s regular speaking voice has changed, her Goku voice could remain the same.

The lady who voices Bart Simpson has kept the same. So it is possible to keep the same voice over years.

Same way spongebob sounds the same after all this time.

I doubt the voice change from 60 to 80 will be all that drastic.

@ragingloli I agree, and for a woman it would be even less drastic.