The Hero Wars are traditionally said to have begun in 1625 with the Dragonrise and to have ended in 1655, and is said to have lasted thirty years. For comparison, the Grand Pendragon Campaign (from when Arthur pulls the sword from the stone to the final battle at Camlann) is from 510 to 565. Another good source of comparison – Alexander became king in 336 BC, died in 323 BC, and his dynasty eliminated in 309 BC.
The Hero Wars are long enough that folk fighting in the final battle might have been born during it – but short enough that folk who fought in the first battle might fight in the last.
Just as a soldier who might have fought at Chaeronea in 338, where Alexander’s dreams began, might have fought at the Battle of Ipsus in 301, where they all ended.
And so our adventurers, who might have fought their first battle at Auroch Hills, Pennel Ford or Dangerford, could possibly be alive to fight the Last Battle, in a world that is almost unrecognisable from its start, in part because of their deeds.
The Pendragon Campaign is just a bit too long for folk present at the start to be expected to be there at the end (they’d be 70 or 80 years old, which is very tough in Pendragon!). But the Hero Wars, it is perfectly possible for your 21 year adventurer of 1625 to be a powerful 51 year old in 1655.
Although being dead is more likely.