Imperial Guard is made up of PDF, far as I know. Bit more life experience there.
Not always. Some of regiments do it this way (Cadians, and by extension lots of other regiments, because for some random reason Cadians are the templates for Guardsmen, just like Ultramarines, durr), but they may as well be schooled to be Imperial Guardsmen from start, either from childhood or the best ones might be choosen to be Guardsmen while rest becomes PDF, they might be convicts, they might get in by lottery or maybe they volunteer, maybe they were born into a warrior caste where all of them become Guardsmen, or maybe they were collected straight from street. There are also other ways, like firstborn from Vostroya or whatever comes to mind really.
Though I thought Space Marines grabbed recruits around the ages of 12-13.
Kind of. Recruits between 10-14 years old are preffered because genetic implants have higher chance of failing if recipent is older, but there have been cases of Marines that haven't started the whole process until they were 18. The whole thing usually takes around 6-8 years anyway, so...
Which makes the whole 'Blood Duel' stuff a bit more cringe-worthy.
...when the Blood Duel could happen (the recruit is ready to be a full Space Marine), he is 18 at least, but even then that's going to take a bit longer because they are going through scout (they are full Space Marines in terms of genetics and implants, they just haven't recived Power Armour yet) phase first.