why do they keep saying that Goku is underage if he is over 500 years old?

his body and mind are still that of a child. When he was found by Sanzo he was still the same age he had been when he was last in heaven. He neither aged nor gained experience in the 500 years he was locked away. Once he was free he started to age again and is currently only 18 (●'◡'●)ﾉ

That makes sense now thank you