I referred to this in my other post, but the real issue is employability. I graduated with my full-time MBA from a top 30 school at the age of 43. I was a top applicant: top 5% in my class according to my professors, 730 GMAT, 3 strong internships while in school, very personable, yet I couldn't get an interview. I was just too "old" for most of the companies recruiting. It shouldn't have been an issue: I was an engineer, got laid off, started a business and built it up to be very successful before realizing I loved strategy and helping others. I wanted to get back to school and got my MBA. It was a great experience: many of my good friends are in their 20's and I don't see an age gap. Most of them would tell you I am a great person to have in class: I work hard, have tons of real-world experience, and have absorbed a ton of knowledge over the years. I was always asked to be on multiple teams, so I must have been a good teammate! That isn't the issue whatsoever. The companies recruiting are the problem: they don't want to deviate from their lane and the predictable nature of hiring the same type of person. As in my other example, if they hire the 29-year old Stanford grad and he doesn't work out, it is just "win some and lose some-cost of doing business". If they hire me, they will hypothesize that
1) I may not like working with/under someone younger than me (even though I did it during grad school)
2) I may quit and go do something else
3) I don't have as much drive as a 29-year old without commitments
4) I have already been successful and may quit after a few months as I will always want to be an entrepreneur.
These reasons are totally bunk, but as I said in the other post, nobody ever got fired for buying IBM.
I got into a smaller elite general strategy consulting firm by being in the right place at the right time; it is a small team and they don't want to hire green candidates that take 2 years to get up to speed. If I need to go in front of a CEO, I need to be polished and confident, and usually 29-year olds don't fit the bill. I owned a client-facing business for a decade and have those skills. I didn't get the job through my MBA program though: every lead there was a dead-end and my advisor kept telling me "you are one of the very top people here-why don't you have a job?" Ageism never occurred to him, but I dealt with it at EVERY larger firm. I was able to get to an interview as they couldn't tell my age on the application, but once we started talking, I was actually asked "well, how will you be able to work under someone younger?". How is that not a negative leading question?
So yes, you can do it, but an MBA has less value from a recruiting perspective the older you are. It will probably be a smaller company: not AT&T's Leadership Dev. Program which caps at 33 years old. Your employer will know that they are getting a "deal" at your post-MBA salary and that you will be able to take on responsibility.
I also would recommend a 2-year program if you can swing it. The immersion is very, very valuable and I made many good friends in my program. I didn't ever view the age as an issue; 40 isn't old anyways! Not to mention I was still one of the fittest guys in my class and if someone had to guess my age, it would be 37 or so.
I hope this helps. My grandfather was laid off at 57 and could never find another senior position, even though he was sharp and the type of person that everyone loved. Ageism really sucks. I hired many retired people in their 60's looking for something fun to do, and boy were they a bargain! I never considered age to be a negative.