In the online discussion groups I frequently see statements like “Young singers should sing lots of Mozart”. That may be true if you’re a soprano or a bass. However, he wrote almost nothing for contraltos and lower mezzos, and most of what he wrote for tenors is very hard to sing. Certainly the vast majority of teen-age tenors should NOT sing Mozart yet.
Main Point: It is hard for tenors because a high proportion of the notes in these arias are in the passaggio (roughtly D4-G#4), with not much rest in between the hard passages.
Larynxes have more similarities than differences across the voice types. However, most men have much larger larynxes than most women, which means that the pitch area around F#4 needs far more time to be coordinated and easy in a tenor or baritone voice than in a soprano voice. In a tenor or baritone, F#4 is in a part of the voice that is in a high part of the range that requires a lot of muscular energy and finesse. A parallel situation: a novice trumpet player can play a C5 easily, but a novice trombonist cannot. When the instrument is built bigger, higher pitches are harder. And doubly hard if they are in a major muscular transition zone.
While Puccini and Donizetti don’t seem to have much in common, their tenor arias don’t hang out in the passaggio as consistently as Mozart’s. The 19th century Italian arias have other challenges, like higher ranges and heavier orchestration, which most people can identify as not appropriate for beginners. It is a little less easy to learn to identify the challenges of Mozart for young men.
Most of Handel’s tenor arias have conservative ranges, and only stay in the passaggio briefly. That is why they are good for young voices. It is not because of time period (Bach and Gluck wrote some nasty-hard stuff for tenor), but because of the composition of the pitches and note values. So when someone says “No Mozart, only baroque arias at first”, that is also not a complete answer.