“Kids grow up so fast these days” - grandparents from every age.
In epic fantasy or historical fiction it can sometimes be tricky to decide how old someone must be before they realistically fight, and how old is too old. History teaches us that there’s a pretty enormous age range for fighting.
Alexander the Great
Alexander, as well as winning every battle he ever fought, also has the distinction of starting very young. In fact, he was just 17 when he commanded the cavalry at the Battle of Chaeronea. His father led the Macedonian army, and Alexander destroyed the elite Theban Sacred Band.
The reassertion of Macedonian hegemony over the Hellenistic world began when Alexander was 19, and the conquest of the Persian Empire occurred when he was in his 20s. Despite his youth he was strategically and tactically astute, and held the respect and loyalty of men old enough to be his father or even grandfather.
Hannibal accompanied his father Hamilcar to Iberia when he was just a boy, and learnt there how to be a soldier. When Hamilcar was killed his son served under Hasdrubal the Handsome, who was assassinated, before taking command himself. Hannibal’s glory years during the early part of the Second Punic War were in his 30s. After the war ended he led Carthage for a short time, before going on the run as an anti-Roman mercenary. During his time he still showed he had tactical ability, not least by the brilliant use of snakes in a naval battle (he had them collected, put into pots and then hurled onto the decks of enemy ships). He died in his 70s, by his own hand.
Caesar is best known two campaigns, the Gallic Wars and Civil Wars. The former occurred when he was in his 40s, and the latter in his 50s. Despite perhaps seeming a little old by the end of it there was never a question of him using age as an excuse to evade the hardships of soldiering. Indeed, on the battlefield he enjoyed great success and won both wars. Not unlike a British political party in the Commons, his adversaries may have been in front of him, but his enemies were behind him.
The Silver Shields
Not a man but an army unit. Believed to have been the hypaspists (elite foot soldiers comparable to the Companions on foot who served under Alexander) they became a critical and feared force during the wars of the Diadochi that ensued after Alexander’s death. Despite their relatively advanced age (hard to be certain but 50s and 60s seems likely) they were not an enemy one wanted to see on the battlefield, and their expertise (and morale-boosting presence) ensured they could command an above average fee from their paymaster.
Lysimachus and Seleucus
These two men were both Companions of Alexander, and both were to live more than twice as long as their famous leader. Although lesser known than the names above, both held sway over tremendous swathes of territory. Seleucus had the lion’s share of what had been the Persian Empire, and Lysimachus held most of Thrace and Asia Minor. The two men fought for their entire lives (on the same side initially, but after the Battle of Ipsus, see below, they squabbled amongst themselves). By the time they met for the final climactic Battle of Corupedium both men were in their 70s.
However, even Lysimachus and Seleucus were younger than old Antigonus. He became, for a time, the predominant successor to Alexander. However, his power was such that four rivals (Cassander, Lysimachus, Seleucus and Ptolemy [debatably]) united to oppose him. At the Battle of the Five Kings (the Battle of Ipsus) he faced a combined army approximately equal to his own. Despite being 81, he commanded his forces along with his son Demetrius Poliorcetes, and was killed by a javelin when his son led the cavalry too far from the battle and was unable to return and save his father due to Seleucus’ elephants (cavalry are terrified of elephants). [Demetrius’ cavalry also included a young cousin of Alexander the Great, called Pyrrhus].
So, just taking a fairly narrow band of history from about the mid-4th century BC to the mid-1st century BC there are clear examples of leaders on the battlefield ranging in age from 17 to over 80. Although the older ones may not have fought personally, it seems highly likely that those in their 50s and 60s were still doing so.