It's never too late to go back to school. If you're over 40, chances are you are more mature and more confident in yourself and your goals than you were when you were 20. If going back to school is something you need in order to advance toward where you want to be, there's no reason why you shouldn't be able to do it successfully. You may face some different challenges when you're an older and arguably wiser adult going back to school, but nothing is insurmountable.
Review the admissions criteria for the program you're interested in, especially any required prerequisites. Many programs require prerequisites to have been completed within a limited amount of time before the application deadline, so even if you took the prerequisites in the past, there's a chance you'll need to repeat them. Try to speak with an admissions counselor to find out what the situation is at the school in question.
Decide what you want to do about employment. It's possible that you might be able to study while continuing to work, especially if the program you want to attend is available via distance learning or as night school. Talk to your boss about going down to part time while you're studying; this may be especially possible if you're going back to school to advance in your current profession.
Look into financing options if you will be leaving your existing job. Several scholarships and student loan programs exist for so-called re-entry students returning to college in adulthood. The admissions counselor at the college you want to attend might be able to point you toward these programs.
Choose your program carefully. Research what you're getting into in terms of a social environment. It's important to choose a program carefully based on factors such as the job outlook and earning potential. Visit the campus, perhaps during a lunch hour, to get an idea of what it will be like to be among the other students.
Send in your application and wait to hear back. Once you have your admissions letter in hand, try to network with other older adults studying the same program so you can compare notes and share experiences. It can be easier to face the stresses of student life when you have appropriate support.