Gonzalo's words mean that the experience of living in a strange country without a working knowledge of the common language often causes one to regress (return to a former, less developed state) from adulthood to childhood.
To illustrate his meaning, Gonzalo relates the experience of his father to us. Due to his very limited knowledge of English, Gonzalo's father is reduced to smiles and mumbles in his interactions with society. He has to rely on Gonzalo to make phone calls, to relay messages to their landlady, and to purchase items in stores where English is the predominant language of commerce. Gonzalo tells us that 'He got younger. I got older.' Like a child, Gonzalo's father has to rely on Gonzalo to perform typically adult errands and tasks.
Gonzalo's mother's Tio Juan has a similar experience. Gonzalo tells us that in Guatemala, Tio Juan was 'the oldest man in his pueblo,' but in America, he essentially became a 'little baby.' This is because Tio Juan cannot understand enough English to watch TV, interact with others, or find work. After Tio Juan loses his way in town and ends up standing in front of a beauty parlor, Gonzalo is tasked with baby-sitting his grand-uncle. It is another illustration of Gonzalo's words "the older you are, the younger you get when you move to the United States."