Immediately after the Majin Buu Saga Goku is only 30. You would expect him to be older but this is because because Goku was dead a total of eight years during DBZ (for one year after Raditz battle, and for seven years between Cell and Majin Buu) and hence he did not age during these periods. Interestingly this makes Goku only 12 years older than Gohan during the Majin Buu Saga.