Different strokes for different folks, different cultures, upbringing, maturity, etc... I've been a "serial monogamist" since I was 17. I can honestly look back at my mindset and different experiences and say - "wow - if only I'd grasped the concept of poly way back in my early 20's, things would've been much easier". I don't think there's any problem with younger people identifying as poly, as long as it's not just an excuse to fuck around.
In a similar vein to the first sentence there - I know people in their mid 30's with several children who are less qualified to be in a relationship than most 19 year olds. I also know my little brother met his GF when they were both 15, she was his first gf and he her 3nd ever bf, and they've been very happily married for 10 years now, just turned 30.
So it's going to vary by person.
In the U.S., our culture is hard to comprehend even if you're here; everything sexualized and sensationalized - but so much is so conservative and so much lip service paid to values and morals. The two can co-exist - I am all for morals and values and want to see more of it! - but it's like America has it backwards. We liberalize the wrong shit and stick in the mud on the stuff we should be moving past. As a result, teens are raised in a culture where everytig is about sex, unless you're very conservatively raised dating more than a few times is synonomous with having sex, so by extension if you're dating around, you're sleeping around - which isn't cool by some bass akwards social ideology still pervasive here. You are expected to settle down with a bf or gf very young now. If you don't, either something is wrong or you're a slut. :shrug: