Is the compensation of an entry level pilot much lower than that of a more experienced one?

An entry-level pilot will make significantly less than an experienced pilot. Some regional airlines are paying as low as $20,000/year for year 1 as a first officer when that same pilot has paid close to $100,000 for training.
There are other programs slowly becoming available that can offset the cost of training and put you in a better position starting than pilots of the last generation.
JetBlue has started a pilot program for students with 0 experience.
If you are willing to fly unmanned aircraft, you will be compensated much better from day 1.

Unfortunately Adrian is pretty correct. The airline industry hasn't yet realized that their pilot shortage will continue for as long as their pay doesn't allow their pilots to survive. Though you do have the magic few that will deal with it just because they love flying, but the industry is seeing less and less of those people every day. However, certain countries (IE China) are starting a very booming aviation industry, and are looking for pilots with all levels of experience.

As for how long it will take you, the easiest route is becoming a flight instructor. Depending on the company, you can earn as little as 250 hours a year, and as high as 1000 hours a year. 1500 total hours is the magic number. It allows you to be eligible for an ATP license, that is a requirement now for all air carriers (Delta, American Airlines, etc.) Depending on how long it takes you to earn that coveted ATP, figure an additional 5 - 10 years, depending on the need of the industry to be getting paid good money. However, if you're looking for 35-50K a year, it will probably only take you 3-5 years past the ATP. Good luck

Adrian and Stephen have both provided excellent answers.
Now, pretty much in any career, an entry level position will pay considerably less than one with experience. How long it takes to advance is entirely up to you. What you put in, is what you get out. In aviation you will hear "Your attitude will determine your altitude!". Work hard, network and always keep an eye on your goals. Remember that there are different kinds of aviation as well, military, commercial or general aviation. Each one has its own rewards and path to success. Try not to compare your success to anyone else's, each individual has different ideas and experiences.
Make your own path and stick to your dreams. Good luck