I was reading Fire & Blood and in there Aegon III got married to Daenaera Targaryen who was 6 years old.
Was this the youngest highborn match in history or did someone take an even younger bride?
Ermesande Hayford is the youngest bride we know of. She was probably two years old when she was married to Tyrek Lannister.
Speaking specifically of Aegon III, he was not that old himself. He was 11 when he married his cousin Jaehaera Targaryen (Who was 8). After her death, he was 13 when he married Daenaera Velaryon. A completely negligible difference in ages. Besides, his first wedding was political to bind the Greens and Blacks together after horrific civil war. His second wedding was simply because his sisters brokered the match and because he liked Daenaera more than other maidens at the Ball. Both weddings were obviously not consummated (1 never at all since Jaehaera died, and second presumably much much later even after Daenaera became an adult since the King hated being touched even by her and kept separate chambers).