Quite often (as in real life), your career will not go the way that you planned and you will have to take another option. Such is part of the fun of Traveller. In the original Traveller, it used to be that getting ejected from a career often meant death. Mongoose softened that blow somewhat (but you can still suffer serious injury).
After you have all been through your careers, we will be picking from a list of extra skills appropriate to the genre, just to ensure that your group isn't completely crippled (in case no one becomes a pilot, for example, Pilot will appear on the skill list).
Unlike a lot of games, where you begin as a rank amateur, in Traveller, you have already lived part of your life. What age you begin at is up to you (determined by how many terms you take). The more terms you take, the more skills (and the more benefits) you can earn, but... if you remain in the workforce too long, you stand a risk of losing physical attributes due to ageing (and of course, there is always the possibility of injury). You decide how much risk you wish to take.
As a matter of practicality, I recommend that no one go over six terms in any one career. Each term is four years long, and ageing rolls begin at age 34 (your character begins at 18 years of age). Staying in for more than six terms really stretches believability (a two-star general has decided to leave retirement and go adventuring?).