Do airlines prefer younger pilots after a cpl?
29 is not too late for them. It might be too late for you, depending on where you are in your life now.

First off, a CPL won't cut it to fly for most Part 121 or Part 135 outfits, including major airlines, cargo haulers etc. As of 2013, the requirements to get on the flight deck of a commercial airliner include an ATPL, which requires a minimum of 1500 hours (1000 if you attend a four-year flight school). 

A CPL, with instrument rating and an endorsement to ferry passengers for hire, would allow you to fly for a company like NetJets, or for a corporate or government jet fleet, though you'll face stiff competition for those jobs; they're fairly cushy and well-paid for the required experience, and you'll find a lot of ex-military and ex-airline pilots beating down the door. You'd also be able to fly foreign-registered aircraft for a non-US airline, as long as the jurisdiction hasn't adopted a similar rule as the FAA's.

When you get your ATPL, with only the 1500 hours and no other airline experience, you'll likely land your first gig as First Officer on a regional route. This is your toehold into a career in flying, and to be perfectly honest if you don't absolutely love to fly I would tell you to do anything else, especially if you've already started a life (married, kids, mortgage). The average starting salary for a regional FO is about \$22,500 per year, which for a 40 hour work week is a meager \$10.75 an hour. You would literally make more working full-time as a barista in Sea-Tac Airport serving coffee to the pilots than flying the puddle-jump between there and Portland. And that's just the average; you can expect half the currently flying pilots with less than 1 year experience to make less than that. You are virtually guaranteed to make more money doing anything else that requires more than a high school diploma.

It gets better, but it takes a career to do it. Senior captains flying the big widebodies on major transcontinental and intercontinental routes make six figures, in excess of \$175k in some cases. Most of this is the airlines trying to retain their most experienced pilots for just a little longer by giving them the most profitable routes for which they can pay the pilots more. The other part is making up for the peanuts they pay you in your early years.

For the majority of your career, your entire life will fit in a carryon and/or a garment bag; flying regionals, you'll make the same flight five to ten times a day and have a good chance to get back to your home city, but on regional pay you'll be hard-pressed to afford more than a single-bedroom flat. As your experience grows, your promotions will naturally lead to longer flights on bigger jets, with the best-paying routes being intercontinental flights where you'll sleep on the plane two nights out of three. Many pilots have month-to-month arrangements with their landlords to rent a single room to crash in and store anything that doesn't fit in their flight bag. This will be your life for 20-30 years.

It is extremely common for newly retired pilots and their spouses to invest in a "pilot's house"; while most retirees are downsizing from the house they raised kids in, most pilots are finally able to settle down in one location for the first time in their lives, and are buying McMansions in cash at about that same age, with savings from the last few years of their flying career.

All in all, flying sounds like a glamorous, "see the world" job, but you have to be sure you can live with the downsides. Your spouse will essentially be a single parent (if you have kids), you'll be living in a small apartment for most of your life and will move several times, requiring your spouse to have a career that allows easy relocation, and you might only be home every other day if not less often, depending on the airline's scheduling model; it's fairly common especially among LCCs to run "end-to-end" routes instead of "hub and spoke", and that could mean you'll start in New York, run hops through Chicago, Atlanta and Dallas with your eight hours allowable flight time per day, then sleep in Dallas and fly the reverse route the next day back to New York.

Just to make a direct comparison, my wife has a cosmetology license (haircuts, nails, spa treatments) in Texas. To get that license required a 2-year course of study and 1500 hours total instruction and guided practice. Her first year with that license, she made about $33000, almost 50% more than a first-year airline pilot. The economics of the airline industry (and the regulations in the cosmetology industry) are bass-ackwards.