Age has always seemed like a really horrible benchmark for level to me. I guess if you have ABSOLUTELY nothing that you care about in terms of a random person walking down the street maybe that's a good half-assed way to work it... but if you don't care anything about them and it's just some random person that means nothing to the game, why does it even matter what level they are? For some reason any random schmuck that's spent their life possibly sitting around doing nothing, picking potatoes from a field, shoveling out horse stalls, etc,. has gained levels in, what? Anything? Simply for breathing X number of years. Yeah, no. Just plain silly.
First, I'd decide what that individual actually is; Farmer, merchant, city guard, adventurer, whatever.
Then I'd gauge their level by what the person has actually done. Sure you might be a city guard, but for how long? Maybe you shoveled the horse stalls in the garrison for years first.
Lvl 1 = No experience
Lvl 2 = Beginner
Lvl 3 = Novice
Lvl 4 = Apprentice
Lvl 5 = Journeyman
Lvl 6 = Expert
Lvl 7 = Master
The other thing to consider is, does it really matter? Not as it what level they are, but as in is that actually going to come into play?
First the combat outlook. If they aren't going to be in a fight, it just doesn't matter. If they are, I'd refer back to the above.
For non-combat, if the characters go to town and ask a Armor-smith to repair their armor, are you actually going to roll dice?
The way I'd look at it is I'm not going to waste my time with that unless I actually want the Armor-smith to have the potential not help the party.
And if I do want the Armor-smith to fail and not help the party... am I actually going to roll for that, or just tell them he failed because it's a plot device?