Age is only relevant to me for one reason: consent.
Beyond that, you're a noob because you lack experience (the very thing that makes you level up!). A person can remain a level 1 fighter soldier for years for a lot of reasons. Perhaps the lands have been in relative peace during your tenure. Perhaps you've got a Captain who doesn't like you and keeps putting you on janitor duty. (heh, duty). Maybe the life of a soldier has become ill-fitting, thus you left the service without advancing your training, perhaps you were discharged. Perhaps advanced training is limited to a select few and you haven't made the cut.
Against Flexor's advice I would not caution against deep backstories for low-level characters. This will often lead to disinterest in a character, with them being little more than a mechanical incarnation designed to "play a game" and not result in good role-play. If you don't know who your character is, how can you play them properly?
I would instead caution against backstories that build your character up to be something they can't actually start out as. A level 1 fighter is essentially a grunt in a regular set of armed forces, or might be a little higher but instead come from a bumpkin town where level 1 represents extreme skill. If you write your character (or if anyone at your table does this) to be the Captain of the City Watch, having served in 4 wars and personally protected the king....unless "max level" of your world is level 5, it's highly unlikely that your character could reasonably have done those things without gaining any levels at all.
I've played a paladin in numerous editions and a "level 1 paladin" is sort of an odd beast since it's mentioned they go through years of training. But that's years of training. That's why you have an 18 str. That's why you have a 16 int. That's why you have a 15 cha. You weren't born with those qualities, you refined them over the years of training you did. When you hit "level 1" as a paladin, that's the first moment you've been accepted in your holy order.
Honestly I would say that groups which require training (Paladins, Fighter/Soldiers, Wizards, Clerics, monks) make more sense to start at level 1, given that these classes often belong to structured institutions which pace your advancement. Rogues, Rangers, Druids, Bards, Barbarians, Sorcerers, Warlocks make less sense to start at level 1 without that change representing essentially a massive mid-life crisis.