There is never an ideal age to join an accounting firm or business. There are certain recommendations that I would make that might advance your career more quickly. Since you are interested in working for an accounting firm, I might suggest going into a public accounting firm right out of college and during your time there get your CPA license. Depending on the career you want in public (e.g. the level of the job), you should stay with it and strive for positions higher up that may include senior associate or manager/ senior manager or even higher. If you decide that along the way, you want to explore other possible career paths, the CPA credential can and will help you do that.
Age is never a deciding factor . . . your education, credentials, work experience and work ethic will lead you to the career you want.