Complex sentence: The second line was formed by the principes, who were drawn from men in their later 20s or early 30s, and the last line was made up by the triarii, who were the oldest and (supposedly) most experienced men in the army.
Split into simpler sentences: The second line was formed by the principes, who were drawn from men in their later 20s or early 30s. The last line was made up by the triarii, who were the oldest and (supposedly) most experienced men in the army.