Youth, young, youngster, teenager, youthfulness So, just having a look at the following words and definitions:

青少年 - n. teenager, youth
年轻人 - n. youngster
年轻 - adj. young
青春 - n. youth; adj. youthful

Am I right in thinking that a 年轻人 is maybe up to 13/14, and that a 青少年 is a teenager (so 14-17)?

And what is the difference, would you say, between 年轻 and 青春? Google images seems to bring up modern pictures of young people for 年轻 and then photos of young people from the 70s/80s with 青春. Not entirely sure why...
年轻人 refers to people younger than the speaker, usually people under 30.

少年 refers to people around 13-18.

青年 refers to people around 18-29.

青春 means the time period of or the experience of the age between 12-29. Often it is retrospective, as when you are in mid-age, you might say:回忆青春岁月/悔不该青春虚度, etc. Newspapers may write: 祖国今日之富强有赖于上一辈人挥洒青春. So perhaps it is the reason you get 80s photos