I recently tried out for American Idol and didn't make it past the 1st cut. This was my third time. I really can sing, everyone who hears me thinks that I have a beautiful voice. I turn 29 on tomorrow, and I'm wondering if I am too old to continue trying for a singing career.

No, you are not too old. Some artists aren't appreciated until they lived a full life. Don't risk everything to be a singer, but don't give up on a love and appreciation for your talent so long as your heart is still in it.

XenoSapien

First NO, American Idol is a real marketing joke often really worst singers make it past the first cut for the laugh value in adds latter. And it can even be the type of style of your music.

My wife used to run the number one music studio in Atlanta GA, we had "Star Search" actually call us to send them people to try out.
And from it while a few get their carrerrs going though these shows.

And I will be honest, only maybe 1/10 of one percent ever get to be world famous singers, but 1000's become career singers making a living at it.
They work on cruise ships, they do background music, they do small clubs and the such. If you want it, you have to go out and get bookngs and sing and sing and sing.

No, you are not too old for even beginning a singing career. In one of the groups that I am the pianist, the singer is in her sixties. She is the only singer for our group and I also know that ours is not the only group for which she is the singer. We gig on a regular basis and travel throughout the upper Midwest of the U.S. We perform on a cruise once a year that is free for us.