This is one of the great dramatic monolgues in English. It’s easy to be carried along by the speaker’s undimmed enthusiasm for exploration (mental or physical) and his reluctance to give into old age. The last two lines are justly famous. But in this poem, as in the best of Browning’s, what is being said is undercut by how it is said. If you pay attention, the poem is having its cake and eating it; admiring the exuberant old explorer, while allowing you to see his arrogance and selfishness.