I still remember how I travelled to a big city as a teenager to watch the film in the cinema, most probably due to a very positive review. The effort was not in vain, for Tadzio’s elegant restraint and his boyish body made a lasting impression on me.
Andresen's statement “I was just 16” seems to refer to his later stay in Cannes during the film festival. So the age range of our Tadzio figure goes from 15 to 14, 13 and 10:
Thomas Mann described in Death in Venice “a long-haired boy about fourteen years old”.
Katia Mann about the real Tadzio:
All the details of the story, beginning with the man at the cemetery, are taken from experience … In the dining-room, on the very first day, we saw the Polish family, which looked exactly the way my husband described them: the girls were dressed rather stiffly and severely, and the very charming, beautiful boy of about 13 was wearing a sailor suit with an open collar and very pretty lacings. He caught my husband's attention immediately. This boy was tremendously attractive, and my husband was always watching him with his companions on the beach. He didn't pursue him through all of Venice—that he didn’t do—but the boy did fascinate him, and he thought of him often… I still remember that my uncle, Privy Counsellor Friedberg, a famous professor of canon law in Leipzig, was outraged: “What a story! And a married man with a family!”
Wikipedia: The boy who inspired “Tadzio” was Baron Władysław Moes, whose first name was usually shortened as Władzio or just Adzio. […]
Moes was born on November 17, 1900 in Wierbka, the second son and fourth child of Baron Aleksander Juliusz Moes. He was aged 10 when he was in Venice, significantly younger than Tadzio in the novella. Baron Moes died on December 17, 1986 in Warsaw and is interred at the graveyard of Pilica, Silesian Voivodeship.
https://en.wikipedia.org/wiki/Death_in_Venice