I've heard it said that companions are locked into a certain power level based on what level you are when they enter your service rather than having a set level or scaling with you, is this true?

Perhaps, but maybe its an ingame mechanism for eventually losing followers over time...

I really doubt that, it seems kind of ridiculous. I'll decide when I'm done with a companion.

Follower stats are generated based on your level when you first enter their area of the game. So if you want strong Followers avoid their city until you are high level.

http://www.uesp.net/wiki/Skyrim:Followers Scroll down to follower stats for explanation
http://elderscrolls.wikia.com/wiki/Follower_%28Skyrim%29#Notes Scroll down to Bugs for explanation

Wow that is [censored] ridiculous, followers are going to be a complete waste of time then. Nevermind.

You can fix them by using the disable / enable trick. Look for the instructions on this page:

http://www.uesp.net/wiki/Skyrim_talk:Followers

Some followers though, I have noticed that they max out at level 30.

To me it seems that many followers are seriously bugged. Like Farkas, a master of heavy armor (he can train you up to skill-level 100) , who when inspected via console only shows a heavy armor skill-level of 15...

On PC many of these issues will be fixed by modders - but that sadly doesn't help people who play on the xbox 360 or PS3