quattuor et viginti

These are cardinal numbers: they can function like adjectives, in that they can directly modify substantive nouns-like heads as attrubutes, except that they often have no adjectival endings. So this phrase is best considered an attributive phrase modifying annos.

In the broadest sense, the main clause is the entire sentence. In the narrowest sense, it is only "Interea ad Hispanias, ..., P. Cornelius Scipio mittitur", though what follows is an appositive ("filius...") to "P. Cornelius Scipio"; you might consider appositives as such to be part of the constituent they are apposed to.

vir Romanorum omnium et sua aetate et posteriore tempore fere primus

This is a second appositive to "P. Cornelius Scipio".

quattuor et viginti

These are cardinal numbers: they can function like adjectives, in that they can directly modify substantive noun-like heads as attributes, except that they often have no adjectival endings. So this phrase is best considered an attributive phrase modifying annos.

In the broadest sense, the main clause is the entire sentence. In the narrowest sense, it is only "Interea ad Hispanias P. Cornelius Scipio mittitur", though what follows is an appositive ("filius...") to "P. Cornelius Scipio"; you might consider appositives as such to be part of the constituent they are apposed to.

vir Romanorum omnium et sua aetate et posteriore tempore fere primus

This is a second appositive to "P. Cornelius Scipio".