You're never too old to start a voiceover business. I get calls all the time from talents younger than I am, thinking they're too old to do voiceovers. This could not be further from the truth. There is an abundance of genres that require mature voices. Commercials, medical narration, Promos, documentaries and audiobooks!
Let's take an eLearning narration project as an example. If this is a series of training modules for a new medical device, a client will rarely hire someone in their twenties to do this particular job. More times than not, this project will go to a voice actor in their forties or above.