***** NOT A TEACHER *****
(1) I believe that you are 100% correct: It is time for you to choose one occupation and concentrate on that.
(a) Many young people do not know what occupation they want, so they try different jobs. "To settle down into a
career" is to stop going from job to job. To choose one and spend your whole life (?) doing the best that you can at
that particular job.
(2) If a young person's parents tell him/her that s/he should "settle down," it means:
(a) Choose one occupation now.
(b) Choose one boy-, girlfriend.
(c) Get married.
(d) Have children.
(e) Buy a house (and spent the next 20 -30 years paying for it).
(f) Pay taxes to the government.
(g) Play with your grandchildren one day.
(h) Grow old and then ...
(3) Of course, if a teacher tells her class to "settle down," it means:
Stop talking and playing! Pay attention to me!
(4) "Settle down" means a million things -- depending on the context (situation).