Is there a point to try to get companions as soon as possible?

Do companions have a fixed levels when I first meet them in game or do they receive experience based on main character's experience when they are added to the roster? Should I try to acquire companions as soon as possible and then finishing all the side quests/clearing areas to give them more experience? For example if I rush search for Bastila and then clear sidequests will she have more experience when I will finish Taris?

Once you reach level 7 any companion you get starts at level 6, but gets enough experience to level to one level below you. Before level 6, the companion is one level below you with one level up not chosen. So it does not really matter. At low level you will get slightly more choice in how you customise your characters, but there is no reason to delay leveling beylnd level 8.

There is one experience aspect not related to companions that is relevant for the decision whether to delay leveling (which will have impact on your companion level before this point):

After you leave Taris, you will start gaining levels as a Jedi class. Any levels not yet assigned will be assignable as a Jedi at this point. Some powerplaying builds will recommend leaving Taris with a couple of levelups banked so you have more powerful jedi skills at this point.