Goku’s exact age is never revealed, but he’s at least over 100. The following information can give a general idea of his age and how old he was when certain events happened in the series.
As a child, Goku lived with Grandpa Gohan on Mount Paozu . He then spends his adolescence training under Master Roshi to control his ki . At some point during their training, Krillin was introduced to them (he was roughly 8-12 years older than Master Roshi). Years later, both Goku and Krillin turn 18 at a party stating that “eighteen is going through an awkward phase that most people go through right before adulthood”.