As young characters grow up, we all know they start low and grow up. Old school an 18 was rare indeed, and if you put a +2 on it for a 20 that was phenomenal. Using a couple of famous guys, Schwarzenegger and André the Giant, I'd guess a Str of 18 and 20 respectively. Trying to put realize to fantasy is always a trap for me.
Also, would you actually allow the numbers to exceed 18 or 20 as they level up?
The question is, would any GMs out there limit the lvl 1 stats to 18 for their party?
Somehow it always seems mind boggling to me to have a party of basically teenagers starting out with such incredible numbers. True, they are heroes, but I'm sure even Hercules wasn't as buff as some of the guys at my table. :P Which makes me wonder what his stats would be like as a demigod.
In modern times most people do not engage in hard physical labor since early childhood. Many characters in the game will have done this. In medieval times they started training early, a warrior type would start training by the age of 7. The only thing that comes close would be athletes. Some football players get pretty bulked up even in high school. Some high school football players are well over 6’, 250#.
Old-school stats were 3d6 straight up, but you always got phenomenally lucky rollers (or cheaters) with multiple 18s. An 18 strength would even get you a percentil roll on top of that, topping out at 18/00. There was no way for a player to get 19 or 20 for a stat.
But that's not what you're asking, is it.
In PF initial stats and raising stats has nothing to do with age, well, at least not unless you actually drift into old age - and those rules are largely negative for physical stats. No, stat raising has everything to do with levels, +1/4 levels, IIRC.
Initial stats depend either on point buy (10/15/20/25 point buy, or whatever the DM wants) or on dice, frequently 4d6/drop lowest and assign.
What all this might have to do with the real world is beyond me. Guess I need to go up a couple of levels and raise my INT. <g>
With the exception of Strength, all the PF attributes are utterly abstract. They attempt to quantify the unquantifiable. As such, the numbers can be interpreted in many ways, particularly since they're all also aggregates of several things, many times unrelated to one another.
I also let the players decide their character's age, so they may or may not be teenagers (and I don't use the aging adjustments).