What do you understand by the following statements?

Yet in both these boyish faces there was a seriousness which was far beyond their years.

When the author saw Nicola and Jacopo shining and polishing shoes outside the hotel, he was surprised and said to them that he thought they sold fruit for a living. Nicola told him that they did many kinds of jobs like taking the tourists to Juliet's tomb and other tourist attractions. The author told Nicola to take them to the tourist places. The author observed Nicola and Jacopo carefully and thought that even though they were very innocent, their faces reflected seriousness and maturity that should be possessing in their later years.