He married at around 25-30, normal for that time because any royal father with an eligible daughter wanted to see some battles and conquests from a potential SIL before entering into an alliance-just good business...
History says Alexander’s whole family was a pack of royal jackals who fought and murdered everyone and each other to get/keep the throne-his mother was so bad that she was actually executed for treason/murder. His legitimate son-by his wife Roxana-was born about 2-3 months after Alexander’s death. Roxana was from around what is now Afghanistan. After being guarded for years, the kid was murdered-along with his mom-at 14-he was about to inherit the throne-by a relative who really wanted it for himself-it is a wonder the kid lived that long. Alexander’s murdering Ptolemy relatives brought all that Greek culture to Egypt, too...
Alexander didn’t need to marry a royal daughter. He could have married someone from a leading Macedonian family. Philip and Alexander were obviously very talented individuals but left a lot to be desired as human beings—very bloodthirsty when it served their interests. The Greeks were not taken in by the propaganda of a war of revenge because of Xerxes’ invasion (Macedonia had been on Persia’s side in 480). The Athenians generally had a poor opinion of Alexander.