I am inquiring about what age you think any of the large consulting firms would deem too old to interview a potential candidate. I am a 39 year old physician that works in a small practice as an anesthesiologist.
My business experience is limited, however I am a part owner of our 28 employee business and prior to medical school I worked on a human genomics business development platform at one of the largest biotechnology firms. I am aware that some of the companies have ADPs for those with MDs/PhDs.
Age is generally not a factor. What is a factor is your career accomplishments relative to your age. So if you're 39, it is expected that would have accomplished a lot more impressive things in your career than an applicant who is 25 years old.
One of my colleagues at McKinsey was probably in his mid 40's when he joined as an Associate / entry level consultant. He was one of the higher performing associates.
Here was his background:
BS, MS in science from a top Ivy league school.
PhD in Astrophysics from top 3 Ivy school
Fully tenured profession of astrophysics at Princeton
In my book, that's an impressive background relative to the age.
So if you're older, that's not a problem... but the expectation is you've been accomplishing really impressive things for all the years of your career. Basically the firms don't like to see any "coasting" in your career. They want to see a continuously upwards (steep slope) trajectory to your career and the older you are, the higher up the curve they expect to see.
So that's the general rule of thumb, firms really like the "Wow" factor on the consulting resume. It's not necessary, but the Wow factor definitely gets people's attention as plenty of people with "pretty good" backgrounds get in too (I personally fell in the latter category, but keep in mind that in the recruiting brochures all the profiles are of the people with the "wow" backgrounds... it's definitely skewed).