I started becoming aware as well by my late teens but not much.
For example, I re-connected with a childhood friend for a time at 18 and she kept blowing me off when I would call by asking me to call back in the next 15 minutes because she this this or that to do. It took me three times before realizing that she did not want to talk to me and that she was giving me the run around. In fact, it helped me realized the first clue that someone is not interested like that.
Things did not start to click until I was in my early 20's since I had the mind of a teenager and most adolescents start to reason at the next 5 years of their lives go by. I know mine did.
I am 31 now and think like I am 25-26. I can now very read cues when someone is either not interested in me by different hints of excuses. I can also tell the different between a broken promise when a fly by night comment about getting together.
I can even read them in the case of my friends and other people by watching how they interact with my friends by their body language, the pitch of their voice, and whether they talk with my friends much at all.
-A broken promise is something that someone says they are going to do constantly by saying as if they are committed and then changing their mind at the last minute or do not follow through, whereas a fly by night is where someone says something like, "Yeah, let's do lunch next week, call me."