Hollywood is perhaps the most cruel industry out there. Nearly all actors struggle for life or until they give up. Rejection is a daily occurrence. The odds of success are horrible. It's common sense that even people that start in adolescence are unquestionably to succeed. So that scares someone that's 25 or 28 years old and is just starting.
Many people in the acting industry say that it takes 10-20 years to "make it". So if your in your mid to late twenties you could be waiting until your middle-aged. Brad Pitt was just two credits away from graduating from college when he started acting. So I would say he was approximately 22-23. Johnny Depp started when he was twenty-one. Dennis Quaid dropped out of college in Texas and moved to LA when he was twenty. Mel Gibson joined the school school when he was 19. Renee 'Zellwegger picked up acting in college. Eric Bana was a licensed bartender before jumping into acting. The oldest beginning actor I've found is Gerard Butler. The star of 300 started acting when he was twenty-five.
You could break it down by gender. Men are not as hopeless as women when it comes to starting acting at an older age. People like Brad Pitt and George Clooney have agreed on-screen and are still considered hot stars. Even the largest female stars reach a certain age and get discarded from Hollywood. Again Hollywood is a cruel industry. If you're a man I would not get too excited about the gender advantage, your odds are still remote. Older actors should look on the bright side. Instead of kicking yourself for not starting at a younger age, realize that you would not have much of a chance even if you started at age five. The answer to the question is simple. Your odds are worse if you start at a later age, but it's still possible that you can have success.
If you decide that you want to pursue an acting career your just going to have to work harder. You should learn to network. Some of the largest stars made it because they knew the right people. Luck is maybe the largest factor. You also should get good training. Stars like Brad Pitt, Halle Berry and Charlize Theron all studied under Ivana Chubbuck. There's still hope if you're a little older and want to be an actor even if you're in your twenties.