Yeah.With the current rate of levelling, they might well still be 20 years old when they hit level 20...
That is why I think of classes as combat experience. They are fighting styles whether fighting with weapons or spells.
A person − if they are fortunate − can go thru an entire lifetime without ever seeing combat.
But others − who are less fortunate − can go thru periods of intense combat.
In D&D, the leveling advancement happens during the combat-heavy adventures. But the times between adventures might stretch out across decades before reaching level 20.
To get to level 20 is something like 200 encounters.